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Jean le Rond d’Alembert

Jean Le Rond d’Alembert

A course in two strands: the life of the abandoned child who became the voice of the French Enlightenment, and the mechanics, waves, fluids and analysis he left to Euler, Lagrange and Laplace.

Born 16 November 1717, Paris Died 29 October 1783, Paris Wave equation first in print, 1747 Level advanced undergraduate
A plucked string, from \(y=\tfrac12[\varphi(x-ct)+\varphi(x+ct)]\). Time runs downward; the two travelling halves (thin lines) add to the string (bold). At \(t=L/c\) the shape returns inverted and reversed.
Lesson 00

How to use this course

Jean le Rond d’Alembert is remembered by two audiences that rarely meet. Historians of ideas know him as co-editor of the Encyclopédie and author of its Discours préliminaire. Physicists and mathematicians know his principle of dynamics, his solution of the wave equation, his paradox of fluid resistance and his ratio test. This course treats both, and shows how the same taste for clear first principles runs through his science and his philosophy.

Prerequisites for Part II
  • Single- and multivariable calculus, including partial derivatives and the chain rule.
  • Newtonian mechanics of particles and rigid bodies (torque, angular momentum, moment of inertia).
  • Ordinary differential equations at the level of a first course.
  • For Lessons 06 and 08, complex numbers and the idea of an analytic function.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements in modern notation, derivations and exercises with folded solutions. D’Alembert wrote in the language of the 1740s: geometric ratios, infinitesimals and long verbal arguments. The lessons translate his results into today’s notation and say plainly where his own arguments had gaps.

Reference

Timeline

1717Born 16 November in Paris to Claudine-Alexandrine Guérin de Tencin; left the next day on the steps of the church of Saint-Jean-le-Rond. Raised by Mme Rousseau, a glazier’s wife.
1726Death of his father, the artillery officer Louis-Camus Destouches, who had paid for his upbringing.
c. 1729Enters the Jansenist Collège des Quatre-Nations, aged 12.
1735Bachelor of arts.
1738Qualifies as an advocate; in 1739 tries medicine.
1739July: first communication to the Académie des Sciences, on errors in Charles Reyneau’s Analyse démontrée.
1741May: admitted to the Académie des Sciences as adjoint astronome, after several failed attempts.
1743Traité de dynamique: d’Alembert’s principle.
1744Traité de l’équilibre et du mouvement des fluides.
1746Associé géomètre of the Paris Academy; elected to the Berlin Academy; memoir containing his proof of the fundamental theorem of algebra.
1747Berlin Academy prize for Réflexions sur la cause générale des vents. Memoir on the vibrating string: the wave equation and its general solution.
1748Fellow of the Royal Society of London.
1749Recherches sur la précession des équinoxes.
1751First volume of the Encyclopédie, with his Discours préliminaire.
1752Essai d’une nouvelle théorie de la résistance des fluides. Frederick II offers him the presidency of the Berlin Academy; he declines.
1754Elected to the Académie française. Recherches sur différens points importans du système du monde (to 1756).
1757–58The article “Genève” provokes a crisis; he gives up the co-editorship of the Encyclopédie.
1761First volume of the Opuscules mathématiques (eight volumes, to 1780).
1763Three months with Frederick II at Potsdam; declines the Berlin presidency again.
1765Serious illness; moves to the house of Julie de Lespinasse. Pensionnaire of the Paris Academy.
1768Finds the young Pierre-Simon Laplace a teaching post at the École Militaire. Ratio test in the Opuscules, vol. 5.
1772Perpetual secretary of the Académie française.
1776Death of Julie de Lespinasse.
1783Dies 29 October at his lodgings in the Louvre, aged 65. Buried in an unmarked grave in the Porcherons cemetery.
Part I · Formation
Lesson 01

A foundling, 1717–1729

D’Alembert was born in Paris on 16 November 1717. Some sources give the 17th, the day he was found. His mother was Claudine-Alexandrine Guérin de Tencin, a former nun released from her vows who later ran one of the most influential salons in Paris. She did not want the child, and the day after his birth he was left on the steps of the small church of Saint-Jean-le-Rond, beside Notre-Dame. The church gave him his first name. He never received any acknowledgement from her.

His father was Louis-Camus Destouches, an artillery officer who was abroad at the time of the birth. On his return Destouches traced the child and placed him with a foster mother, Mme Rousseau, the wife of a glazier. D’Alembert lived in her modest house for nearly fifty years and remained attached to her all his life. Destouches paid for his upbringing and, when he died in 1726, left him an income large enough to live on simply. The Destouches family continued to look after his education.

The circumstances of his birth shaped his public character. He had no family name, no rank and no inheritance beyond a small annuity. Everything he gained came through talent and through the institutions of learning. His later insistence that merit, and not birth, should decide places in the Republic of Letters had a personal source.

Lesson 02

College, law and medicine, 1729–1739

At about twelve he entered the Collège des Quatre-Nations, founded by Cardinal Mazarin and strongly Jansenist in outlook. He was registered there as Jean-Baptiste Daremberg; the name d’Alembert appears a little later, and nobody is sure where it came from. The college gave a classical and theological education. Its mathematics course, taught by a professor named Carron, followed the lectures of Pierre Varignon. His teachers hoped he would become a theologian, and he did write a commentary on St Paul’s Epistle to the Romans. He came away instead with a lasting distrust of theology and of Cartesian metaphysics.

He graduated as bachelor of arts in 1735. Like many young men without a career, he studied law and qualified as an advocate in 1738. He then spent a year on medicine. Neither held him. He taught himself mathematics from borrowed books, mostly the works of Isaac Newton and of the French Leibnizian school, and he later said he discovered for himself results he afterwards found in print.

First papers

In July 1739, aged 21, he sent the Académie des Sciences a memoir pointing out errors in Charles Reyneau’s Analyse démontrée, then a standard textbook of the calculus. A second memoir in 1740 drew the attention of Alexis Clairaut, only four years older and already an academician. The work was competent rather than brilliant, but it showed exactly the skill the Academy needed: fluency in the new calculus and the willingness to apply it to physical problems.

Lesson 03

The Academy, 1739–1746

After several unsuccessful applications in quick succession, d’Alembert was admitted to the Académie des Sciences in May 1741 as an adjoint astronome, the lowest of its ranks. The next five years were the most creative of his life. In 1743 he published the Traité de dynamique, which reduced the dynamics of constrained systems to statics (Lesson 04). In 1744 he applied the same principle to fluids in the Traité de l’équilibre et du mouvement des fluides, a rival to Daniel Bernoulli’s Hydrodynamica of 1738.

His rise brought conflict. Clairaut, who had welcomed him, became a rival once their interests overlapped in mechanics and celestial mechanics. D’Alembert was quick, sharp-tongued and ready to claim priority, and his writing was often hard to follow. He was nevertheless recognised: in 1746 he became associé géomètre in Paris and was elected to the Berlin Academy, where Pierre-Louis Moreau de Maupertuis was president and Leonhard Euler director of the mathematics class.

The same years opened Paris society to him. Introduced into the salon of Marie-Thérèse Geoffrin in 1746, and later a fixture of the salon of Mme du Deffand, the foundling became a celebrated talker and mimic. It was through these circles that the publisher André Le Breton and the writer Denis Diderot brought him into the project that would make him famous beyond science: the Encyclopédie.

Part II · The mathematics and physics
Lesson 04

D’Alembert’s principle

For the general reader. Newton’s laws tell you how a free particle moves. Real machines are full of rods, strings, hinges and surfaces that constrain motion, and the forces they exert are unknown in advance. D’Alembert found a way to remove those forces from the problem. He turned every question in dynamics into a question of equilibrium, which mechanics already knew how to answer.

Goals
  • State d’Alembert’s principle in Lagrange’s form with virtual displacements.
  • See why ideal constraint forces drop out.
  • Derive Lagrange’s equations from the principle and use it on simple machines.

The idea of 1743

In the Traité de dynamique d’Alembert considered the motion each body would take if it were free, and split it into two parts: the motion it actually takes, and a motion that is “destroyed” by its connection with the other bodies. His principle says that the destroyed motions alone would leave the system in equilibrium. He stated it in words and with geometric constructions, and he deliberately avoided talking about force as a cause, which he regarded as obscure. Joseph-Louis Lagrange later combined it with the principle of virtual work and gave it the form used today.

Modern statement

Consider \(N\) particles with masses \(m_i\) and positions \(\mathbf r_i\), acted on by applied forces \(\mathbf F_i\) and constraint forces \(\mathbf R_i\). A virtual displacement \(\{\delta\mathbf r_i\}\) is an infinitesimal change of configuration consistent with the constraints at a fixed instant. The constraints are ideal if \(\sum_i\mathbf R_i\cdot\delta\mathbf r_i=0\) for every virtual displacement: rigid rods, inextensible strings, frictionless surfaces and rolling without slipping all qualify.

D’Alembert’s principle (Lagrange’s form)

For a system with ideal constraints, the actual motion satisfies

\[ \sum_{i=1}^{N}\big(\mathbf F_i-m_i\ddot{\mathbf r}_i\big)\cdot\delta\mathbf r_i=0 \]

for every virtual displacement \(\{\delta\mathbf r_i\}\). The term \(-m_i\ddot{\mathbf r}_i\) is the “inertial force”; the principle says applied and inertial forces are in equilibrium in the sense of virtual work.

Derivation. Newton’s second law gives \(m_i\ddot{\mathbf r}_i=\mathbf F_i+\mathbf R_i\). Dot with \(\delta\mathbf r_i\) and sum. The constraint terms vanish by ideality, leaving the principle. Its power is that the unknown \(\mathbf R_i\) never appear.

From the principle to Lagrange’s equations

For holonomic constraints, write \(\mathbf r_i=\mathbf r_i(q_1,\dots,q_n,t)\) with independent generalized coordinates \(q_k\), so \(\delta\mathbf r_i=\sum_k\frac{\partial\mathbf r_i}{\partial q_k}\delta q_k\). With kinetic energy \(T=\tfrac12\sum_i m_i|\dot{\mathbf r}_i|^2\) and generalized forces \(Q_k=\sum_i\mathbf F_i\cdot\partial\mathbf r_i/\partial q_k\), the identities \(\partial\dot{\mathbf r}_i/\partial\dot q_k=\partial\mathbf r_i/\partial q_k\) and \(\frac{d}{dt}\partial\mathbf r_i/\partial q_k=\partial\dot{\mathbf r}_i/\partial q_k\) turn the principle into

\[ \sum_k\Big(\frac{d}{dt}\frac{\partial T}{\partial\dot q_k}-\frac{\partial T}{\partial q_k}-Q_k\Big)\delta q_k=0. \]

Since the \(\delta q_k\) are independent, each bracket vanishes. If \(Q_k=-\partial V/\partial q_k\), then with \(L=T-V\) these are the Euler–Lagrange equations \(\frac{d}{dt}\frac{\partial L}{\partial\dot q_k}=\frac{\partial L}{\partial q_k}\). This is the route Lagrange took in the Mécanique analytique of 1788.

Exercise 4.1Atwood’s machine: masses \(m_1>m_2\) hang from a light inextensible string over a frictionless massless pulley. Use the principle to find the acceleration without computing the tension.

Show solution
Measure \(x\) downward for \(m_1\); then \(m_2\) moves up by \(x\). A virtual displacement is \(\delta x\) for \(m_1\) and \(-\delta x\) for \(m_2\). The principle gives \((m_1g-m_1\ddot x)\delta x+(m_2g+m_2\ddot x)(-\delta x)=0\), so \(\ddot x=\dfrac{m_1-m_2}{m_1+m_2}\,g\). The tension does no virtual work and never appears.

Exercise 4.2A simple pendulum of length \(l\) makes angle \(\theta\) with the downward vertical. Derive its equation of motion from the principle.

Show solution
Virtual displacements are \(\delta\mathbf r=l\,\delta\theta\,\hat{\boldsymbol\theta}\). Gravity has component \(-mg\sin\theta\) along \(\hat{\boldsymbol\theta}\), and the acceleration has tangential component \(l\ddot\theta\). So \((-mg\sin\theta-ml\ddot\theta)\,l\,\delta\theta=0\), giving \(\ddot\theta+\frac gl\sin\theta=0\). The rod tension is normal to \(\delta\mathbf r\) and drops out.

Exercise 4.3A uniform solid cylinder of mass \(M\) and radius \(a\) rolls without slipping down a plane inclined at angle \(\alpha\). Find its acceleration.

Show solution
Let \(s\) be the distance travelled; rolling forces the rotation angle to be \(s/a\). A virtual displacement \(\delta s\) moves the centre by \(\delta s\) and turns the cylinder by \(\delta s/a\). The principle for a rigid body reads \((Mg\sin\alpha-M\ddot s)\,\delta s-I\frac{\ddot s}{a}\cdot\frac{\delta s}{a}=0\) with \(I=\tfrac12Ma^2\). Hence \(\ddot s=\dfrac{g\sin\alpha}{1+\tfrac12}=\tfrac23 g\sin\alpha\). Friction at the contact point does no virtual work because that point is instantaneously at rest.
Lesson 05

The vibrating string and the wave equation

For the general reader. Pluck a guitar string and its shape changes in a complicated way. In 1747 d’Alembert wrote down the equation the string obeys and found every solution: the motion is always the sum of two shapes, one sliding to the right and one to the left, each keeping its form. This was the first partial differential equation of physics to be solved in general, and the argument it started shaped analysis for a century.

Goals
  • Derive the one-dimensional wave equation for a stretched string.
  • Prove that every \(C^2\) solution is a sum of two travelling waves, and derive d’Alembert’s formula.
  • Understand the dispute with Euler and Daniel Bernoulli about which functions are allowed.

The equation

A string of linear density \(\rho\) under tension \(T\) lies along the \(x\)-axis; let \(y(x,t)\) be its small transverse displacement. The net vertical force on a short piece \([x,x+\Delta x]\) is \(T\,[y_x(x+\Delta x,t)-y_x(x,t)]\approx T\,y_{xx}\,\Delta x\) for small slopes, and Newton’s law gives

\[ \frac{\partial^2y}{\partial t^2}=c^2\,\frac{\partial^2y}{\partial x^2},\qquad c=\sqrt{T/\rho}. \]

Brook Taylor had found the fundamental frequency of a string in 1713 by assuming a particular shape. D’Alembert’s memoir, Recherches sur la courbe que forme une corde tendue mise en vibration, written for the Berlin Academy in 1747, was the first to state the equation in this partial-derivative form and to solve it without assumptions on the shape. The operator \(\square=\partial_t^2-c^2\nabla^2\) is still called the d’Alembertian.

Theorem (d’Alembert, 1747)

Let \(y\in C^2(\mathbb R^2)\) satisfy \(y_{tt}=c^2y_{xx}\). Then there are \(C^2\) functions \(F,G\) with \(y(x,t)=F(x-ct)+G(x+ct)\). The solution with initial data \(y(x,0)=\varphi(x)\), \(y_t(x,0)=\psi(x)\), where \(\varphi\in C^2\) and \(\psi\in C^1\), is unique and given by

\[ y(x,t)=\tfrac12\big[\varphi(x-ct)+\varphi(x+ct)\big]+\frac1{2c}\int_{x-ct}^{x+ct}\psi(s)\,ds. \]

Proof. Put \(\xi=x-ct\), \(\eta=x+ct\). By the chain rule \(\partial_x=\partial_\xi+\partial_\eta\) and \(\partial_t=c(\partial_\eta-\partial_\xi)\), so \(y_{tt}-c^2y_{xx}=-4c^2\,y_{\xi\eta}\). The equation becomes \(y_{\xi\eta}=0\): \(y_\xi\) does not depend on \(\eta\), so \(y_\xi=f(\xi)\) and \(y=F(\xi)+G(\eta)\). For the initial value problem, \(F+G=\varphi\) and \(c(G'-F')=\psi\). Integrating the second relation, \(G-F=\frac1c\int_0^x\psi+K\); solving the two linear equations for \(F\) and \(G\) gives the formula.

Fixed ends

For a string fixed at \(x=0\) and \(x=L\) the conditions \(y(0,t)=y(L,t)=0\) force \(F(-ct)=-G(ct)\) and a similar relation at \(L\). Equivalently, extend \(\varphi\) and \(\psi\) from \([0,L]\) to odd, \(2L\)-periodic functions on \(\mathbb R\) and use the formula unchanged. The motion is then periodic in time with period \(2L/c\), so the fundamental frequency is

\[ f_1=\frac{1}{2L}\sqrt{\frac{T}{\rho}}, \]

the law of Marin Mersenne, now derived from mechanics.

The controversy

D’Alembert insisted that the initial shape be given by a single analytic expression, so that the equation made sense everywhere. In 1748 Euler replied that any shape one can draw, such as a plucked string with a corner, must be admissible, since the travelling-wave formula still describes it. In 1753 Daniel Bernoulli argued from physics that every motion is a superposition of the modes \(\sin(n\pi x/L)\cos(n\pi ct/L)\). Euler objected that a sum of sines could not represent an arbitrary curve; d’Alembert rejected both views. Lagrange joined the debate in 1759. The questions it raised, what a function is and which functions are sums of trigonometric series, were settled only with the work of Joseph Fourier, Peter Gustav Lejeune Dirichlet and Bernhard Riemann. In modern terms, Euler’s corner shapes are weak solutions, and Bernoulli’s series converge to them.

Exercise 5.1Check directly that d’Alembert’s formula satisfies \(y(x,0)=\varphi(x)\) and \(y_t(x,0)=\psi(x)\).

Show solution
At \(t=0\) the integral is over an empty interval and the bracket is \(2\varphi(x)\), so \(y=\varphi\). Differentiating, \(y_t=\tfrac c2[-\varphi'(x-ct)+\varphi'(x+ct)]+\frac1{2c}[c\,\psi(x+ct)+c\,\psi(x-ct)]\), which at \(t=0\) equals \(0+\psi(x)\).

Exercise 5.2A string fixed at \(0\) and \(L\) is released from rest (\(\psi=0\)) with shape \(\varphi\). Show that at time \(t=L/c\) its shape is \(y(x,L/c)=-\varphi(L-x)\).

Show solution
Let \(\Phi\) be the odd \(2L\)-periodic extension. Then \(y(x,L/c)=\tfrac12[\Phi(x-L)+\Phi(x+L)]=\Phi(x-L)\) by periodicity, and \(\Phi(x-L)=-\Phi(L-x)=-\varphi(L-x)\) by oddness, for \(x\in[0,L]\). The string is turned upside down and reflected left to right; after another \(L/c\) it is back where it started. The hero figure shows this.

Exercise 5.3A guitar string has vibrating length \(0.65\) m, linear density \(1.2\times10^{-3}\) kg/m and tension \(70\) N. Find the wave speed and the fundamental frequency.

Show solution
\(c=\sqrt{70/0.0012}=\sqrt{58\,333}\approx241.5\) m/s. Then \(f_1=c/(2L)=241.5/1.30\approx186\) Hz, close to the F♯ below middle C.
Lesson 06

The fundamental theorem of algebra

For the general reader. Every polynomial equation, however high its degree, has a solution among the complex numbers. D’Alembert gave the first serious attempt at a proof in 1746. His key idea, that a polynomial’s size can always be pushed lower unless it is already zero, is the heart of the shortest proof taught today. In France the result is still called the d’Alembert–Gauss theorem.

Goals
  • State and prove d’Alembert’s lemma.
  • Combine it with a compactness argument to prove the theorem.
  • See where d’Alembert’s own argument fell short.
Fundamental theorem of algebra

Every non-constant polynomial \(p(z)=a_nz^n+\cdots+a_0\) with complex coefficients has a root in \(\mathbb C\). Consequently \(p\) factors as \(a_n\prod_{j=1}^n(z-z_j)\).

D’Alembert’s lemma

Let \(p\) be a non-constant complex polynomial and \(z_0\in\mathbb C\) with \(p(z_0)\ne0\). Then there is \(z\in\mathbb C\) with \(|p(z)|<|p(z_0)|\).

Proof. Expand about \(z_0\): \(p(z_0+h)=p(z_0)+b_kh^k+b_{k+1}h^{k+1}+\cdots+b_nh^n\), where \(b_k\ne0\) is the first non-zero coefficient after the constant term (one exists because \(p\) is not constant). Choose \(\omega\) with \(\omega^k=-p(z_0)/b_k\); complex \(k\)-th roots always exist. For real \(t\in(0,1)\),

\[ p(z_0+t\omega)=p(z_0)\,(1-t^k)+O(t^{k+1}),\qquad\text{so}\qquad |p(z_0+t\omega)|\le|p(z_0)|(1-t^k)+Ct^{k+1}, \]

which is less than \(|p(z_0)|\) once \(t<|p(z_0)|/C\).

Completing the proof

Because the leading term dominates, \(|p(z)|\to\infty\) as \(|z|\to\infty\) (Exercise 6.2). Choose \(R\) so that \(|p(z)|>|p(0)|\) for \(|z|>R\). The continuous function \(|p|\) attains a minimum on the compact disc \(|z|\le R\) at some \(z_0\), and this is a global minimum. By the lemma, \(p(z_0)\) cannot be non-zero. So \(p(z_0)=0\).

What d’Alembert did and did not prove

D’Alembert’s argument appeared in a memoir on integral calculus for the Berlin Academy for 1746. He needed the theorem to integrate rational functions by partial fractions, which requires that every real polynomial factor into real linear and quadratic factors. His version of the lemma relied on expanding a branch of the inverse function in fractional power series, a step he did not justify. More seriously, nothing in the mathematics of his time guaranteed that \(|p|\) attains a minimum. Carl Friedrich Gauss criticised his proof, along with those of Euler and Lagrange, in his 1799 doctoral thesis. The proof above, with the minimum taken on a compact set, is essentially the one Jean-Robert Argand published in 1814; full rigour needed the extreme value theorem of the nineteenth century.

Exercise 6.1Carry out d’Alembert’s step for \(p(z)=z^2+1\) at \(z_0=0\). In which direction does \(|p|\) decrease?

Show solution
\(p(0+h)=1+h^2\), so \(k=2\), \(b_2=1\), and we need \(\omega^2=-1\): \(\omega=\pm i\). Then \(p(ti)=1-t^2<1\) for \(0<t\le1\), reaching the root at \(t=1\). Along the real axis \(|p|\) increases; the descent direction is imaginary.

Exercise 6.2Let \(p(z)=z^n+a_{n-1}z^{n-1}+\cdots+a_0\) and \(A=\sum_{j<n}|a_j|\). Show that \(|p(z)|\ge\tfrac12|z|^n\) whenever \(|z|\ge\max(1,2A)\).

Show solution
For \(|z|\ge1\), \(\sum_{j<n}|a_j||z|^j\le A|z|^{n-1}\). So \(|p(z)|\ge|z|^n-A|z|^{n-1}=|z|^{n-1}(|z|-A)\ge|z|^{n-1}\cdot\tfrac12|z|\) when \(|z|\ge2A\).

Exercise 6.3Why does the lemma fail for real polynomials of a real variable? Use \(p(x)=x^2+1\).

Show solution
On \(\mathbb R\), \(|x^2+1|\ge1\) with minimum at \(x=0\), where \(p(0)\ne0\). The proof needs \(\omega^2=-1\), which has no real solution. The existence of \(k\)-th roots of every complex number is exactly what makes \(\mathbb C\) work.
Lesson 07

Linear differential equations

For the general reader. Many of d’Alembert’s physical problems, from strings loaded with beads to the oscillations of the atmosphere, led to systems of linear differential equations. He developed two techniques that every student still learns: how to find a second solution from a known one, and how to split a coupled system into independent pieces by taking the right combinations.

Goals
  • Use reduction of order, usually credited to d’Alembert, to find a second solution.
  • Decouple a constant-coefficient system by the method of multipliers.
  • Recognise multipliers as left eigenvectors and the resulting combinations as normal modes.

Reduction of order

Proposition (reduction of order)

Let \(P,Q\) be continuous on an interval \(I\) and \(y_1\) a solution of \(y''+Py'+Qy=0\) with no zeros on \(I\). Then \(y=v\,y_1\) is a solution if and only if \(w=v'\) satisfies \(y_1w'+(2y_1'+Py_1)w=0\). Hence a second, independent solution is

\[ y_2(x)=y_1(x)\int\frac{e^{-\int P\,dx}}{y_1(x)^2}\,dx. \]

Proof. Substitute \(y=vy_1\): \(y''+Py'+Qy=v(y_1''+Py_1'+Qy_1)+v''y_1+v'(2y_1'+Py_1)\). The first bracket vanishes. The remaining first-order equation for \(w=v'\) is separable: \(w=C\,e^{-\int P}/y_1^2\). The Wronskian of \(y_1,y_2\) is \(y_1^2v'=e^{-\int P}\ne0\), so they are independent.

The method of multipliers

For a system \(\dot x_i=\sum_j a_{ij}x_j\) with constant coefficients, d’Alembert multiplied the equations by constants \(\lambda_i\) and added them, seeking a combination \(u=\sum_i\lambda_ix_i\) that satisfies a single equation \(\dot u=\mu u\).

Proposition

Let \(A=(a_{ij})\) and \(\dot{\mathbf x}=A\mathbf x\). If \(\boldsymbol\lambda^{\mathsf T}A=\mu\,\boldsymbol\lambda^{\mathsf T}\), then \(u=\boldsymbol\lambda^{\mathsf T}\mathbf x\) satisfies \(\dot u=\mu u\), so \(u(t)=u(0)e^{\mu t}\). If \(A\) has \(n\) independent left eigenvectors, the \(n\) combinations determine \(\mathbf x(t)\) completely.

The condition is an eigenvalue problem, \(\det(A^{\mathsf T}-\mu I)=0\), more than a century before matrices existed. For second-order systems \(\ddot{\mathbf x}=-K\mathbf x\) the same idea gives combinations with \(\ddot u=-\omega^2u\): the normal modes of oscillation that d’Alembert, Daniel Bernoulli and Euler used for beaded strings and hanging chains, and that Lagrange made general.

Exercise 7.1Verify that \(y_1=x^2\) solves \(x^2y''-3xy'+4y=0\) on \(x>0\), and use reduction of order to find a second solution.

Show solution
\(x^2\cdot2-3x\cdot2x+4x^2=0\). In standard form \(P=-3/x\), so \(e^{-\int P}=x^3\) and \(v'=x^3/x^4=1/x\), \(v=\ln x\). Thus \(y_2=x^2\ln x\), as expected for a repeated root of the indicial equation.

Exercise 7.2Solve \(\dot x=y,\ \dot y=x\) with \(x(0)=1,\ y(0)=0\) by multipliers.

Show solution
Adding gives \((x+y)^\cdot=x+y\); subtracting gives \((x-y)^\cdot=-(x-y)\). So \(x+y=e^t\) and \(x-y=e^{-t}\), hence \(x=\cosh t\), \(y=\sinh t\). The multipliers \((1,1)\) and \((1,-1)\) are the left eigenvectors with \(\mu=\pm1\).

Exercise 7.3Two equal masses \(m\), each attached to a wall by a spring of stiffness \(k\), are joined by a spring of stiffness \(\kappa\): \(m\ddot x_1=-kx_1+\kappa(x_2-x_1)\), \(m\ddot x_2=-kx_2-\kappa(x_2-x_1)\). Find the normal modes.

Show solution
Adding: \(m(x_1+x_2)^{\cdot\cdot}=-k(x_1+x_2)\), so \(\omega_1^2=k/m\) (masses in step; the middle spring is unstretched). Subtracting: \(m(x_1-x_2)^{\cdot\cdot}=-(k+2\kappa)(x_1-x_2)\), so \(\omega_2^2=(k+2\kappa)/m\) (masses in opposition).
Lesson 08

Fluids, complex functions and the paradox

For the general reader. Why does a moving body feel the resistance of the air or water? In 1752 d’Alembert set out to compute this from first principles for an ideal fluid with no internal friction. He found that the answer was exactly zero, in flat contradiction with experience. On the way he wrote down, for the first time, the equations that link fluid flow in a plane to functions of a complex variable.

Goals
  • Derive the equations of plane potential flow and recognise the Cauchy–Riemann equations.
  • Compute the flow and pressure around a circular cylinder.
  • State d’Alembert’s paradox and its modern resolution.

Plane flow and analytic functions

D’Alembert’s Essai d’une nouvelle théorie de la résistance des fluides (1752) grew out of an entry for a Berlin prize competition on fluid resistance for which no prize was awarded; he blamed Euler’s influence. For a steady plane flow with velocity \((u,v)\) that is incompressible and has no rotation, he obtained

\[ \frac{\partial u}{\partial x}+\frac{\partial v}{\partial y}=0,\qquad \frac{\partial u}{\partial y}-\frac{\partial v}{\partial x}=0. \]

These say that \(u-iv\) satisfies the Cauchy–Riemann equations: it is an analytic function \(w'(z)\) of \(z=x+iy\). The complex potential \(w=\phi+i\psi\) combines the velocity potential \(\phi\) (with \(\nabla\phi=(u,v)\)) and the stream function \(\psi\), whose level curves are streamlines. Euler took up the same relations soon after, and Augustin-Louis Cauchy and Bernhard Riemann made them the foundation of complex analysis.

The paradox

D’Alembert’s paradox

Let a rigid body of finite size be held fixed in a steady, incompressible, inviscid flow that is irrotational and tends to a uniform velocity \(\mathbf U\) at infinity. In three dimensions the net force exerted by the fluid on the body is zero. In two dimensions the drag (the component along \(\mathbf U\)) is zero, and the lift is \(-\rho U\Gamma\), where \(\Gamma\) is the circulation round the body.

D’Alembert found the vanishing resistance in 1752 for symmetric bodies and stated it as a general paradox for the geometers to explain in the fifth volume of his Opuscules (1768). The general proof uses conservation of momentum in a large sphere around the body: the potential decays fast enough at infinity that no momentum flux escapes to infinity. The two-dimensional lift formula is the Kutta–Joukowski theorem of the early twentieth century.

The cylinder

For a circular cylinder of radius \(a\) in a stream of speed \(U\), take \(w(z)=U\big(z+\frac{a^2}{z}\big)\). On \(|z|=a\) the flow is tangential with speed \(2U|\sin\theta|\). Bernoulli’s equation \(p+\tfrac12\rho|\mathbf u|^2=\text{const}\) gives the surface pressure

\[ p(\theta)=p_\infty+\tfrac12\rho U^2\big(1-4\sin^2\theta\big). \]

This is symmetric under \(\theta\mapsto\pi-\theta\): the pressure on the rear face equals that on the front, so it pushes back exactly as hard as it pushes forward. Real flows separate behind the body and leave a low-pressure wake. The resolution came in 1904, when Ludwig Prandtl showed that a thin boundary layer, where viscosity cannot be neglected however small it is, controls separation and drag.

Exercise 8.1For \(w(z)=U(z+a^2/z)\), find the stream function \(\psi\) and show that \(|z|=a\) is a streamline and that the velocity tends to \((U,0)\) at infinity.

Show solution
With \(z=re^{i\theta}\), \(w=U(re^{i\theta}+\frac{a^2}{r}e^{-i\theta})\), so \(\psi=\operatorname{Im}w=U\big(r-\frac{a^2}{r}\big)\sin\theta\), which vanishes on \(r=a\). And \(w'(z)=U(1-a^2/z^2)\to U\), so \(u-iv\to U\).

Exercise 8.2Using \(p(\theta)\) above, compute the drag \(D=-\int_0^{2\pi}p(\theta)\cos\theta\,a\,d\theta\). Where is the pressure highest and lowest?

Show solution
\(\int_0^{2\pi}\cos\theta\,d\theta=0\) and \(\int_0^{2\pi}\sin^2\theta\cos\theta\,d\theta=\big[\tfrac13\sin^3\theta\big]_0^{2\pi}=0\), so \(D=0\). The maximum \(p_\infty+\tfrac12\rho U^2\) is at the stagnation points \(\theta=0,\pi\); the minimum \(p_\infty-\tfrac32\rho U^2\) is at \(\theta=\pm\pi/2\).

Exercise 8.3Show from d’Alembert’s two equations that \(u\) and \(v\) are harmonic, and that \(\phi\) and \(\psi\) are too.

Show solution
Differentiate the first equation in \(x\) and the second in \(y\) and add: \(u_{xx}+v_{yx}+u_{yy}-v_{xy}=0\), so \(\Delta u=0\); similarly \(\Delta v=0\). With \(u=\phi_x,\ v=\phi_y\) the first equation is \(\Delta\phi=0\); with \(u=\psi_y,\ v=-\psi_x\) the second is \(\Delta\psi=0\).
Lesson 09

Precession, nutation and the system of the world

For the general reader. The Earth’s axis does not point in a fixed direction. Like a tilted spinning top, it slowly sweeps out a cone, completing a circle in about 26,000 years, so the pole star changes over the ages. Newton explained the cause but got the details wrong. In 1749 d’Alembert gave the first correct dynamical theory, treating the Earth as a rigid, slightly flattened spinning body pulled by the Sun and Moon.

Goals
  • Compute the averaged torque of the Sun on an oblate Earth.
  • Derive the precession rate and estimate it numerically.
  • Place the 1749 treatise in the lunar and planetary work of Clairaut, Euler and d’Alembert.

The torque on an oblate Earth

Model the Earth as a rigid body with axial symmetry, principal moments \(A=B<C\), spinning with angular velocity \(\omega\) about the symmetry axis, which is tilted at angle \(\varepsilon\) (the obliquity) to the normal of the ecliptic. A body of mass \(M\) on a circular orbit of radius \(a\) exerts a torque whose average over one orbit has magnitude

\[ \langle N\rangle=\frac{3GM}{2a^3}\,(C-A)\,\sin\varepsilon\cos\varepsilon, \]

directed along the line of the equinoxes. It tries to pull the equatorial bulge into the plane of the orbit.

Precession rate (rigid, fast-spinning, orbit-averaged)

Under these assumptions the spin axis precesses about the ecliptic pole, retrograde, at the rate

\[ \Omega=\frac{3}{2}\,\frac{n^2}{\omega}\,\frac{C-A}{C}\,\cos\varepsilon,\qquad n^2=\frac{GM}{a^3}, \]

where \(n\) is the orbital mean motion. The Moon contributes a term of the same form, with its own \(n\) and with \(GM\) replaced by \(G M_{\text{Moon}}\).

Derivation. For fast spin the angular momentum is essentially \(\mathbf L=C\omega\,\hat{\mathbf k}\). The torque is perpendicular to \(\mathbf L\), so it rotates \(\mathbf L\) without changing its length; the tip moves on a circle of radius \(L\sin\varepsilon\). Hence \(\Omega=\langle N\rangle/(C\omega\sin\varepsilon)\), which gives the formula. The fraction \(H=(C-A)/C\approx1/305\) is the Earth’s dynamical ellipticity.

Nutation

The Moon’s orbit is tilted by about 5° to the ecliptic, and its nodes regress with a period of about 18.6 years. The lunar torque therefore varies, and the axis nods about its mean cone. James Bradley announced this nutation from two decades of observations in 1748. D’Alembert’s treatise of 1749 derived both the precession and the nutation from the laws of mechanics. Euler published on the same problem in 1751 without citing him; he later conceded d’Alembert’s priority on this question, and the episode added to their growing hostility.

The Moon and the system of the world

In 1747 Clairaut, Euler and d’Alembert independently found that first-order perturbation theory gave only half the observed motion of the Moon’s apsides, and Clairaut briefly proposed modifying the inverse-square law. Clairaut showed in 1749 that higher-order terms account for the rest. D’Alembert published his own lunar theory and tables in the Recherches sur différens points importans du système du monde (1754–56). The quarrels over priority and method between the three men were bitter, but between them they turned Newton’s geometric arguments into the analytical celestial mechanics of Lagrange and Laplace.

Exercise 9.1Estimate the solar part of the precession. Use \(n=2\pi\) per year, \(\omega=2\pi\times366.26\) per year (the sidereal day), \(H=1/305.5\) and \(\varepsilon=23.44^\circ\). Express the answer in arcseconds per year.

Show solution
\(\Omega=\tfrac32\cdot\frac{2\pi}{366.26}\cdot\frac{1}{305.5}\cdot\cos23.44^\circ\approx1.5\times0.017155\times0.003273\times0.9175\approx7.73\times10^{-5}\) rad/yr, or about \(15.9''\) per year. The Moon is closer, and its contribution is about 2.2 times larger, near \(34.6''\). The total, about \(50.5''\) per year, is close to the observed \(50.3''\).

Exercise 9.2From the observed rate of \(50.3''\) per year, find the period of precession. Explain from the torque formula why a spherical Earth, or an Earth with \(\varepsilon=0\), would not precess.

Show solution
A full circle is \(1\,296\,000''\), so the period is \(1\,296\,000/50.3\approx25\,800\) years. The torque is proportional to \((C-A)\sin\varepsilon\cos\varepsilon\): it vanishes if \(C=A\) (no bulge to pull) or if \(\sin\varepsilon=0\) (the bulge already lies in the orbital plane, so the pull is symmetric).
Lesson 10

Limits, series and chance

For the general reader. The calculus of the eighteenth century rested on “infinitely small” quantities that nobody could define. D’Alembert argued that it should rest on limits instead, and he gave a simple test for when an infinite sum adds up to a finite number. He also wrote on probability, where his scepticism led him to famous mistakes as well as to sharp questions.

Goals
  • Read d’Alembert’s limit definition of the derivative.
  • Prove the ratio test and apply it.
  • Analyse the “croix ou pile” error.

Limits

In the article “Différentiel” of the Encyclopédie (vol. 4, 1754) d’Alembert explained the derivative as the limit of the ratio of finite differences: the ratio \(\frac{f(x+h)-f(x)}{h}\) approaches a value as closely as one pleases as \(h\) shrinks, and that value is the differential coefficient. He wrote that the metaphysics of the calculus lies entirely in the notion of limit. His definition of limit in the article “Limite” was still informal, and Cauchy and Karl Weierstrass gave it its modern form, but the programme was his.

The ratio test

In the fifth volume of the Opuscules (1768), studying the binomial series, d’Alembert used the ratio of consecutive terms to decide convergence.

Ratio test

Let \(a_n>0\). If \(\limsup_{n\to\infty}a_{n+1}/a_n<1\), then \(\sum a_n\) converges. If \(a_{n+1}/a_n\ge1\) for all large \(n\), then \(\sum a_n\) diverges. If \(\lim a_{n+1}/a_n=1\), the test gives no information.

Proof. Choose \(r\) with \(\limsup a_{n+1}/a_n<r<1\). For \(n\ge N\), \(a_{n+1}\le ra_n\), so \(a_{N+k}\le r^ka_N\) and the tail is dominated by the geometric series \(a_N\sum r^k\). If \(a_{n+1}\ge a_n\) eventually, the terms do not tend to 0.

Croix ou pile

In the Encyclopédie article “Croix ou pile” (1754) d’Alembert asked for the probability of getting heads at least once in two tosses of a coin. He argued that the game stops as soon as heads appears, so there are three outcomes: heads at once, tails then heads, tails twice. Two are favourable, so he gave \(2/3\). The standard answer is \(3/4\). He later raised more serious doubts about expectation in the St Petersburg problem and criticised Daniel Bernoulli’s mathematical case for smallpox inoculation, arguing that a long-run average gain need not persuade an individual facing an immediate risk.

Exercise 10.1Use the ratio test to find the values of \(x\) for which the binomial series \(\sum_k\binom{m}{k}x^k\) converges, when \(m\) is not a non-negative integer.

Show solution
\(\Big|\binom{m}{k+1}x^{k+1}\Big/\binom{m}{k}x^k\Big|=\Big|\frac{m-k}{k+1}\Big||x|\to|x|\). The series converges absolutely for \(|x|<1\) and diverges for \(|x|>1\), since the terms do not tend to 0. The endpoints need finer tests.

Exercise 10.2Find the error in d’Alembert’s count for “croix ou pile”, and repair it.

Show solution
The three outcomes are not equally likely. Heads on the first toss has probability \(1/2\), while tails-heads and tails-tails each have probability \(1/4\). Equivalently, list the four equally likely sequences HH, HT, TH, TT; three contain a head. The probability is \(1/2+1/4=3/4\).

Exercise 10.3Show that the ratio test cannot decide between \(\sum1/n\) and \(\sum1/n^2\).

Show solution
In both cases \(a_{n+1}/a_n\to1\), yet the first diverges and the second converges (compare with \(\int dx/x\) and \(\int dx/x^2\)). A finer test, such as Raabe’s, is needed when the limit is 1.
Part III · Life and legacy
Lesson 11

The Encyclopédie and the Academies, 1746–1765

From the late 1740s d’Alembert was co-editor, with Denis Diderot, of the Encyclopédie, ou Dictionnaire raisonné des sciences, des arts et des métiers, responsible for mathematics and the physical sciences. He wrote or revised more than a thousand articles, among them most of the mathematical entries. The first volume appeared in 1751 with his Discours préliminaire. It sets out a genealogy of human knowledge from sensation, and a classification of the sciences under memory, reason and imagination adapted from Francis Bacon. It made him a public figure across Europe.

He was elected to the Académie française in 1754, a rare honour for a man of science. His literary and philosophical essays were collected in the Mélanges de littérature, d’histoire et de philosophie (from 1753), and his Éléments de musique (1752) presented the harmonic theory of Jean-Philippe Rameau.

Crisis and withdrawal

In 1757 the seventh volume carried his article “Genève”, written after a visit to Voltaire. It praised the Genevan pastors for a rational faith close to Socinianism and urged the city to allow a theatre. The pastors protested, Jean-Jacques Rousseau answered with his Lettre à d’Alembert sur les spectacles (1758), and in Paris the attacks on the Encyclopédie intensified until its privilege was revoked in 1759. D’Alembert gave up the co-editorship early in 1758, leaving Diderot to finish the work largely alone, though he continued to supply mathematical articles.

Kings and empresses

Frederick II of Prussia offered him the presidency of the Berlin Academy in 1752 and again later; he refused, and in 1763 spent three months with the king at Potsdam, again declining. Catherine II of Russia invited him to St Petersburg as tutor to her son; he declined that too. He preferred his independence in Paris, his small lodgings and the salons. From the 1750s he was close to Julie de Lespinasse, who ran her own salon from 1764.

DiscussionThe Discours préliminaire derives all knowledge from the senses and orders the sciences by the faculties of the mind. Compare its place for mathematics with the role mathematics plays in the Traité de dynamique, where d’Alembert tries to found mechanics on principles as evident as those of geometry.

Lesson 12

Last years, death and legacy, 1765–1783

In 1765 a serious illness led d’Alembert to leave Mme Rousseau’s house, where he had lived since infancy, and move to Julie de Lespinasse’s apartment; the same year he became a full pensionnaire of the Académie des Sciences. From about then he complained that he could no longer concentrate on long calculations, though the Opuscules mathématiques continued until 1780.

Patron of the next generation

His influence shifted to the people he helped. He corresponded for years with Joseph-Louis Lagrange, whom he championed with Frederick II; in 1766 Lagrange succeeded Euler in Berlin. In 1768 the nineteen-year-old Pierre-Simon Laplace arrived in Paris with a letter of introduction; d’Alembert found him a post teaching mathematics at the École Militaire and pressed his candidacy at the Academy. Nicolas de Condorcet was another protégé, and became his heir and the author of his éloge. In a letter of 1777 he told Lagrange that he could no longer do geometry.

In 1772 he was elected perpetual secretary of the Académie française, and he wrote many of its éloges of deceased members. The death of Julie de Lespinasse in 1776, and the discovery from her papers that she had loved other men, left him grief-stricken. He died of a bladder disease at his lodgings in the Louvre on 29 October 1783, after refusing an operation. As a known unbeliever he was refused burial in the church and was buried in an unmarked grave in the Porcherons cemetery.

Legacy

  • Mechanics: d’Alembert’s principle, through Lagrange’s Mécanique analytique, underlies analytical mechanics.
  • Partial differential equations: the wave equation, its general solution, the d’Alembertian \(\square\), and the debate over functions that led to Fourier analysis.
  • Fluids and complex analysis: the Cauchy–Riemann relations in plane flow, and a paradox that shaped fluid mechanics until boundary-layer theory.
  • Analysis: the limit concept as the foundation of the calculus, the ratio test, and the d’Alembert–Gauss theorem.
  • Astronomy: the first dynamical theory of precession and nutation.
  • Honours: Fellow of the Royal Society (1748), member of the Berlin Academy (1746), Foreign Honorary Member of the American Academy of Arts and Sciences (1781); a lunar crater bears his name.

Final projectChoose one of Lessons 04, 05 or 08. Write a two-page account for a strong undergraduate that states d’Alembert’s result in modern form, works one example in full, and explains one specific gap or controversy in his original treatment and how it was later resolved.

Reference

Connections

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Reference

Sources and further reading

Biography

  1. J. J. O’Connor and E. F. Robertson, Jean le Rond d’Alembert, MacTutor History of Mathematics.
  2. Jean le Rond d’Alembert, Wikipedia; and the French article Jean Le Rond d’Alembert.
  3. J. Pappas and others, Alembert, Jean le Rond d’, Complete Dictionary of Scientific Biography.
  4. T. L. Hankins, Jean d’Alembert: Science and the Enlightenment, Oxford University Press, 1970.
  5. V. Le Ru, Jean Le Rond d’Alembert philosophe, Vrin, 1994.
  6. M. J. A. N. Caritat de Condorcet, Éloge de M. d’Alembert, Académie des Sciences, 1783.

Mathematics and physics

  1. J. d’Alembert, Traité de dynamique, Paris, 1743.
  2. J. d’Alembert, Recherches sur la courbe que forme une corde tendue mise en vibration, Hist. Acad. Roy. Sci. Berlin 3 (1747, publ. 1749).
  3. J. d’Alembert, Recherches sur le calcul intégral, Hist. Acad. Roy. Sci. Berlin 2 (1746, publ. 1748).
  4. J. d’Alembert, Recherches sur la précession des équinoxes et sur la nutation de l’axe de la terre, Paris, 1749.
  5. J. d’Alembert, Essai d’une nouvelle théorie de la résistance des fluides, Paris, 1752.
  6. J. d’Alembert, Opuscules mathématiques, 8 vols., Paris, 1761–1780.
  7. Œuvres complètes de d’Alembert, critical edition, CNRS Éditions, from 2002.
  8. C. Truesdell, The rational mechanics of flexible or elastic bodies 1638–1788, in L. Euler, Opera Omnia II.11.2, 1960.
  9. J. Ravetz, Vibrating strings and arbitrary functions, in The Logic of Personal Knowledge, 1961.
  10. G. Grimberg, W. Pauls and U. Frisch, Genesis of d’Alembert’s paradox and analytical elaboration of the drag problem, arXiv:0801.3014.
  11. H. Goldstein, C. Poole and J. Safko, Classical Mechanics, 3rd ed., Addison-Wesley, 2002 (d’Alembert’s principle; precession of the equinoxes).