A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.

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used only for functions based on gamma, not functions with some obscure relation to gamma
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For questions related to 'elementary' proofs in a technical sense, which has nothing to do with the difficulty of the argument or result. A typical example would be 'elementary' proofs of the Prime Nu…
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Recreational mathematics or puzzles with serious mathematical content. Note that math contest problems are generally considered off-topic.
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Stability theory, including global stability (in dynamical systems, where it can notably be used in combination with ds.dynamical-systems)
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For questions on divisors and multiples, mainly but not exclusively of integers, and related and derived notions such as sums of divisors, perfect numbers and so on.
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For questions that specifically ask for determining a closed form of equations, integrals etc.
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the complexity class consisting of problems whose yes instances can be verified in polynomial time. NP stands for 'nondeterministic polynomial time '.
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the study of varieties endowed with a Poisson structure, which is a certain kind of 2-tensor. This is closely related to symplectic geometry.