A comparison theorem is any of a variety of theorems that compare properties of various mathematical objects.
In the theory of differential equations, comparison theorems assert particular properties of solutions of a differential equation (or of a system thereof) provided that an auxiliary equation/inequality (or a system thereof) possesses a certain property. See also Lyapunov comparison principle
- Gronwall's inequality, and its various generalizations, provides a comparison principle for the solutions of first-order ordinary differential equations.
- Sturm comparison theorem 
- Aronson and Weinberger used a comparison theorem to characterize solutons to Fisher's equation, a reaction--diffusion equation.
In Riemannian geometry it is a traditional name for a number of theorems that compare various metrics and provide various estimates in Riemannian geometry.
- Rauch comparison theorem relates the sectional curvature of a Riemannian manifold to the rate at which its geodesics spread apart.
- Toponogov's theorem
- Myers's theorem
- Hessian comparison theorem
- Laplacian comparison theorem
- Morse–Schoenberg comparison theorem
- Berger comparison theorem, Rauch–Berger comparison theorem, M. Berger, "An Extension of Rauch's Metric Comparison Theorem and some Applications", llinois J. Math., vol. 6 (1962) 700–712
- Berger–Kazdan comparison theorem 
- Warner comparison theorem for lengths of N-Jacobi fields (N being a submanifold of a complete Riemannian manifold) F.W. Warner, "Extensions of the Rauch Comparison Theorem to Submanifolds" (Trans. Amer. Math. Soc., vol. 122, 1966, pp. 341–356).
- Bishop–Gromov inequality, conditional on a lower bound for the Ricci curvatures (R.L. Bishop & R. Crittenden, Geometry of manifolds)
- Lichnerowicz comparison theorem
- Eigenvalue comparison theorem
- See also: Comparison triangle
- Limit comparison theorem, about convergence of series
- Comparison theorem for integrals, about convergence of integrals
- Zeeman's comparison theorem, a technical tool from the theory of spectral sequences
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