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In mathematics or logic, a statement whose validity has been established or proved. It consists of a hypothesis and a conclusion, beginning with certain assumptions that are necessary and sufficient to establish a result. A system of theorems that build on and augment each other constitutes a theory. Within any theory, however, only statements that are essential, important, or of special interest are called theorems. Less important statements, usually stepping-stones in proofs of more important results, are called lemmas. A statement proved as a direct consequence of a theorem is a corollary of the theorem. Some theorems (and even lemmas and corollaries) are singled out and given titles (e.g., Gödel's theorem, fundamental theorem of algebra, fundamental theorem of calculus, Pythagorean theorem).
This entry comes from Encyclopædia Britannica Concise. For the full entry on theorem, visit Britannica.com.