These are things named after Jacques Hadamard (1865–1963), a French mathematician. (For references, see the respective articles.)
Complex analysis
Cauchy–Hadamard theorem – a statement in complex analysis describing the radius of convergence of a power series Hadamard factorization theorem – concerning the factorization of entire functions of finite order The Hadamard product formula for the Riemann zeta function Hadamard gap theorem – concerning the analytic continuation of complex power series Hadamard three-circle theorem – concerning the maxima of holomorphic functions within concentric circles in the complex plane Hadamard three-lines theorem – concerning the maxima of holomorphic functions defined in vertical strips of the complex plane Hadamard's lemma – a result closely related to the first-order term in Taylor's theorem Hadamard regularization Hadamard's gamma function Hadamard multiplication theorem – for the singularities of coefficient-wise product of power series Hadamard's theorem on the radius of meromorphy – for the formula for the m-th radius of meromorphy
Matrix theory
Hadamard matrix – a square matrix whose entries are either +1 or −1 and whose rows are mutually orthogonal Regular Hadamard matrix Hadamard conjecture Complex Hadamard matrix Butson-type Hadamard matrix Hadamard code – a system used for signal error detection and correction based on Hadamard matrices Hadamard's inequality – a bound on the determinant of a matrix whose entries are complex numbers in terms of the lengths of its column vectors Hadamard's maximal determinant problem Hadamard product – for entry-wise matrix multiplication
Differential geometry
Hadamard space – a geodesically complete metric space of non-positive curvature Cartan–Hadamard theorem – a statement in Riemannian geometry concerning the structure of complete Riemannian manifolds of non-positive sectional curvature. Cartan–Hadamard conjecture – an open problem in Riemannian geometry and geometric measure theory concerning the generalization of the classical isoperimetric inequality to spaces of nonpositive sectional curvature Hadamard manifold
Differential equations and dynamical systems
Hadamard's method of descent – a method of solving partial differential equations by reducing dimensions Hadamard parametrix construction – a method of solving second order partial differential equations Hadamard's dynamical system – a type of chaotic dynamical system Hadamard–Rybczynski equation Hadamard variation formula
Cryptography and quantum computing
Hadamard gate – a standard quantum gate that generalizes a coin flip Hadamard test Hadamard transform – an example of a generalized class of Fourier transforms Fast Walsh–Hadamard transform – an efficient algorithm to compute the Hadamard transform Pseudo-Hadamard transform
Real analysis
Hermite–Hadamard inequality – a bound on the mean value of a convex function in an interval in terms of the value it takes at the mid-point and ends of interval Hadamard fractional integral
Probability and statistics
Hadamard derivative Hadamard variance
External links Encyclopedia of Math: Hadamard theorem