These are things named after Jacques Hadamard (1865–1963), a French mathematician. (For references, see the respective articles.)
Theorems, lemmas, and conjectures
Many of these results are sometimes called "Hadamard's theorem". 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 Cauchy–Hadamard theorem, a statement in complex analysis describing the radius of convergence of a power series 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 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's lemma, a result closely related to the first-order term in Taylor's theorem Ostrowski–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 regions bounded by parallel lines in the complex plane.
Mathematical objects and definitions
Hadamard product: entry-wise matrix multiplication an infinite product expansion for the Riemann zeta function Hadamard code Hadamard's dynamical system Hadamard manifold Hadamard matrix Hadamard space Hadamard Transform and Hadamard gate Hadamard variance Hadamard's gamma function