Hadamard may refer to:
- Jacques Hadamard (1865–1963), a French mathematician, whose name is associated with the following topics in mathematics:
- Differential geometry
- Hadamard space, a geodesically complete metric space of non-positive curvature
- Cartan-Hadamard theorem, a result on the topology of non-positively curved manifolds
- 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
- Complex analysis and convexity
- Hadamard three-lines theorem: a bound on the maximum modulus complex analytic functions defined on a strip in the complex plane;
- Hadamard three-circle theorem, a bound on the maximum modulus of complex analytic functions defined on an annulus in the complex plane; closely related to the three-lines theorem;
- Hadamard factorization theorem, a specific factorization of an entire function of finite order, involving its zeros and the exponential of a polynomial
- Ostrowski–Hadamard gap theorem, a result on the analytic continuation of lacunary power series
- Hermite–Hadamard inequality, bounding the integral of convex functions.
- Transform calculus
- Hadamard transform, an example of a generalized class of Fourier transforms
- Fast Walsh–Hadamard transform, an efficient algorithm to compute the Hadamard transform
- Theory of matrices
- Hadamard matrix, a square matrix whose entries are either +1 or −1 and whose rows are mutually orthogonal
- Hadamard code, a system used for signal error detection and correction based on Hadamard matrices
- Hadamard's inequality, a bound on the determinants of matrices.
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