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2. Double factorial - Wikipedia

   en.wikipedia.org/wiki/Double_factorial

   These are counted by the double factorial 15 = (6 − 1)‼. In mathematics, the double factorial of a number n, denoted by n‼, is the product of all the positive integers up to n that have the same parity (odd or even) as n. [1] That is,

3. Complete graph - Wikipedia

   en.wikipedia.org/wiki/Complete_graph

   The number of perfect matchings of the complete graph K n (with n even) is given by the double factorial (n – 1)!!. [12] The crossing numbers up to K 27 are known, with K 28 requiring either 7233 or 7234 crossings. Further values are collected by the Rectilinear Crossing Number project. [13] Rectilinear Crossing numbers for K n are

4. Eulerian number - Wikipedia

   en.wikipedia.org/wiki/Eulerian_number

   The permutations of the multiset {,,,, …,,} which have the property that for each k, all the numbers appearing between the two occurrences of k in the permutation are greater than k are counted by the double factorial number ()!!.

5. Factorial - Wikipedia

   en.wikipedia.org/wiki/Factorial

   TI SR-50A, a 1975 calculator with a factorial key (third row, center right) The factorial function is a common feature in scientific calculators. [73] It is also included in scientific programming libraries such as the Python mathematical functions module [74] and the Boost C++ library. [75]

6. Hyperoperation - Wikipedia

   en.wikipedia.org/wiki/Hyperoperation

   The parameters of the hyperoperation hierarchy are sometimes referred to by their analogous exponentiation term; [15] so a is the base, b is the exponent (or hyperexponent), [12] and n is the rank (or grade), [6] and moreover, (,) is read as "the bth n-ation of a", e.g. (,) is read as "the 9th tetration of 7", and (,) is read as "the 789th 123 ...

7. List of unsolved problems in mathematics - Wikipedia

   en.wikipedia.org/wiki/List_of_unsolved_problems...

   Many mathematical problems have been stated but not yet solved. These problems come from many areas of mathematics, such as theoretical physics, computer science, algebra, analysis, combinatorics, algebraic, differential, discrete and Euclidean geometries, graph theory, group theory, model theory, number theory, set theory, Ramsey theory, dynamical systems, and partial differential equations.