Indicator functions, direct images and inverse images
Translate set operations into computations on functions, and characterise injectivity and surjectivity through direct and inverse images.
Three problem sets per chapter, of increasing difficulty: ★★★ mastering the course, ★★★ standard competitive-exam level, ★★★ the most selective parts. Each problem set takes about 6 hours.
The documents (PDF) are written in French. Titles, objectives and quizzes are translated into English.
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Translate set operations into computations on functions, and characterise injectivity and surjectivity through direct and inverse images.
Prove the Knaster–Tarski fixed-point theorem and apply it to closure operators, Galois connections and inductive definitions.
Prove the two dual decomposition theorems for a finite ordered set, then apply them to divisibility and to sequences of numbers.
Master the computation of trigonometric sums, then compute $\sum_{k=1}^{n-1}\cotan^2\frac{k\pi}n$ and deduce, by squeezing, the value of $\lim_n\sum_{k=1}^n\frac1{k^2}$.
Study a triangle inscribed in the unit circle with complex numbers: orthocentre, Euler line, nine-point circle, Simson line.
Study Möbius transformations of the complex plane, their action on circles and lines, the cross-ratio, then the transformations preserving the unit disc and a “distance” they preserve.
Handle simple and double sums with ease, through the study of the harmonic numbers $H_n=\sum_{k=1}^n\frac1k$.
Prove the main Fibonacci identities by three complementary methods: induction, Binet's formula and binomial coefficients.
Prove the great discrete inequalities (means, Cauchy–Schwarz, rearrangement, Chebyshev) by purely algebraic methods, then Carleman's inequality with its optimal constant $\e$.
Learn the techniques of computation with the floor function and apply them to sums and to counting lattice points.
Measure “how well” a real number can be approximated by rationals, deduce an irrationality criterion and construct a very well approximable real number.
Expand a real number as a continued fraction, establish the properties of the convergents and their optimality, and apply them to $\sqrt2$, the golden ratio, $\pi$ and the Pell–Fermat equation.
Carry out the full study of a sequence defined by $u_{n+1}=f(u_n)$: stable interval, monotonicity, limit, rate of convergence.
Compare algorithms computing $\sqrt a$, $\frac1a$ and $\pi$ by their rate of convergence (linear, quadratic, cubic), and prove these rates.
Establish a general averaging theorem, deduce the Stolz–Cesàro theorem, and use it to obtain the precise behaviour of slowly converging recursive sequences.
Use the great theorems of real analysis (intermediate values, extreme values, Rolle) in varied situations, up to Darboux's theorem.
Find all continuous functions satisfying a functional equation, by the method “values on a dense subset, then continuity”.
Derive the great inequalities of analysis from convexity, study log-convex functions, and prove Karamata's majorization inequality.
Solve first-order linear equations, including at points where the coefficient of $y'$ vanishes, then reduce classical non-linear equations to linear ones.
Fully study the damped and driven harmonic oscillator, a common model in mechanics and electricity, and quantify the resonance phenomenon.
Study qualitatively, without computing them, the solutions of a second-order linear equation with variable coefficient: separation of zeros, comparison, estimates of the gaps between zeros.
Solve classical equations in integers (linear, quadratic, Fermat's equation for exponent $4$) by combining factorisation, congruences and infinite descent.
Study the multiplicative arithmetic functions ($\varphi$, $\mu$, $d$, $\sigma$) and Dirichlet convolution, and characterise the even perfect numbers.
Study the distribution of prime numbers by elementary methods: bound $\pi(x)$ between two multiples of $\frac x{\ln x}$, and prove that there is always a prime between $n$ and $2n$.
Use the vocabulary of groups on concrete transformation groups, and recognise isomorphisms.
Explore, in three families of rings, the remarkable elements (units, idempotents, nilpotents, zero divisors), and construct $\C$ and $\Q(\sqrt2)$ as rings of matrices.
Study the subfields $\Q(\sqrt d)$, their isomorphisms and automorphisms, then prove that the identity is the only ring homomorphism from $\R$ to $\R$.
Compute the powers of a matrix by several methods (binomial formula, annihilating polynomial, change of basis) and apply them to coupled sequences.
Study two iterative methods for solving $Ax=b$, establish a convergence criterion (strictly diagonally dominant matrices) and estimate the rate of convergence.
Study rank-one matrices and “low-rank updates” of an invertible matrix, a central tool of scientific computing (recursive least squares, changing one coefficient).
Construct the Chebyshev polynomials from the formula $T_n(\cos\theta)=\cos(n\theta)$ and establish their algebraic and analytic properties.
Construct the Bernoulli polynomials, establish their symmetry properties, and deduce formulas for the sums $\sum_{k=0}^{m-1}k^p$.
Study the cyclotomic polynomials $\Phi_n$, show that they have integer coefficients, prove that $\Phi_p$ is irreducible, and prove that there are infinitely many primes congruent to $1$ modulo $n$.
Prove binomial identities by combinatorial arguments (double counting, bijections) using lattice paths.
Count the partitions of a finite set and the surjections, and relate these numbers to polynomials and to the exponential series.
Study integer partitions through bijections: conjugation of Ferrers diagrams, Euler's theorem (odd parts and distinct parts), and Euler's pentagonal number theorem via Franklin's involution.
Master Bayes' formula, then study the distribution of the number of fixed points of a random permutation and its approximation by a Poisson distribution.
Study the simple random walk: probability of returning to $0$, reflection principle, distribution of the first return time, and recurrence or transience depending on $p$.
Compare concentration inequalities for sums of Bernoulli variables (Bienaymé–Chebyshev, Hoeffding), and prove the Weierstrass theorem for Lipschitz functions using Bernstein polynomials.
Learn the technique of Taylor expansions and apply it to limits, tangents and asymptotes.
Obtain precise asymptotic expansions of classical sums ($H_n$, $\sum\frac1{k^2}$, $\sum\ln k$, $\sum\frac1{\sqrt k}$) by the “corrected telescoping” method.
Measure the rate of convergence of iterative methods (fixed point, Newton, secant) using Taylor expansions, then accelerate slow convergence (Aitken's $\Delta^2$ process, Richardson extrapolation).
Use the isomorphism between polynomials and values at points to build approximate integration formulas that are exact on polynomials of given degree (rectangle, trapezoidal and Simpson's rules).
Establish the properties of nilpotent endomorphisms: index, sequence of ranks, cyclic bases, reduction to a “Jordan” matrix, and commutant.
Study differentiation $D$, translation $\tau$ and the difference operator $\Delta=\tau-\Id$ on polynomials, the Newton binomial basis, and the “exponential” and “logarithm” relations between them.
Compute the Vandermonde determinant and its variants, then apply it to interpolation, to circulant matrices and to the linear independence of families of functions.
Study various generating sets of $\Sg_n$, relate the number of inversions of a permutation to sorting by adjacent transpositions, and count permutations by number of inversions or cycles.
Construct the resultant of two polynomials, characterise the existence of a common root by the vanishing of a determinant, and deduce the discriminant and elimination methods.
Work with the canonical inner product of $\Mat_n(\R)$ and derive matrix inequalities from it, then interpret linear regression as an orthogonal projection.
Construct the orthogonal polynomials associated with a weight, establish their recurrence relation and the location of their zeros, and deduce the Gaussian quadrature formulas.
Study the approximation of periodic functions by trigonometric polynomials, prove Parseval's identity for Lipschitz functions using the Fejér kernel, and deduce Wirtinger's inequality and the isoperimetric inequality.
Compute limits of sums and products using Riemann sums, and establish classical integral inequalities.
Pass from discrete convexity inequalities to their integral versions, and apply them to bounds on integrals and means.
Prove, by elementary integration methods, that $\pi^2$ is irrational (hence so is $\pi$), and that $\e^r$ is irrational for every non-zero rational $r$.
Determine the convergence of classical integrals, including conditionally convergent ones, and master the comparison tools.
Establish Frullani's theorem and compute classical integrals ($\int_0^{\pi/2}\ln\sin$, $\int_0^\infty\frac{\ln(1+t^2)}{1+t^2}$, …) by elementary manipulations.
Study the Laplace transform of a continuous function, its behaviour at $0$ and at $+\infty$ (Abelian theorems), and prove that it characterises the function.
Master the alternating series test and its bound on the remainder, study “almost alternating” series by asymptotic expansion, and handle classical oscillating terms from competitive exams.
Characterise the convergence of series of rational functions, introduce the function $H(x)=\sum_{n\ge1}\bigl(\frac1n-\frac1{n+x}\bigr)$ that extends the sequence $(H_n)$, and use it to compute sums explicitly.
Prove the Abel–Dirichlet test, study the series $\sum\frac{\e^{\i n\theta}}{n^\alpha}$, then compute the sums $\sum\frac{\sin(nx)}n$, $\sum\frac{\cos(nx)}n$, $\sum\frac{1-\cos(nx)}{n^2}$ using the Riemann–Lebesgue lemma, with applications to $\zeta(2)$ and the identity $\sum\frac{\sin n}n=\sum\frac{\sin^2n}{n^2}=\frac{\pi-1}2$.
Check that a piecewise-defined function is of class $\mathcal C^1$, then solve first-order partial differential equations by a suitable change of variables; recognise the vector fields that are gradients.
Study the curves traced by a point of a circle rolling without slipping on a line or a circle, analyse their cusps and establish some remarkable geometric properties (normal through the contact point, chords of constant length of the cardioid, tangents of the deltoid).
Characterise convex functions of class $\mathcal C^1$ on $\R^2$ by their gradient, then prove the geometric convergence of fixed-step gradient descent for strongly convex functions with Lipschitz gradient, and study the optimal step in the quadratic case.