These are things named after Jacques Hadamard (1865–1963), a French mathematician. (For references, see the respective articles.)

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.
• Hadamard's embedding theorem [ru]

• 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

• Hadamard–Rybczynski equation
• Hadamard's maximal determinant problem
• Hadamard's method of descent
• Hadamard regularization

• Encyclopedia of Math: Hadamard theorem