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Let a spherical triangle have sides of length a, b, and c, and semiperimeter s. Then the spherical excess E is given by

A method for constructing magic squares of singly even order n>=6.

Given a positive integer n, a labeled n-digraph is a digraph whose vertex set is {1,...,n}.

The Labs septic is a septic surface having 99 ordinary double points, which is the maximum number known for any septic surface.

A Fourier series in which there are large gaps between nonzero terms a_n or b_n.

A function that has a natural boundary.

A lag system, introduced by Wang (1963), is a sort of opposite to a tag system. Lag systems allow dependence on more than just the first element, but remove only the first ...

The second-order ordinary differential equation y^('')+k/xy^'+epsilony^'y=0.

A quantity involving primitive cube roots of unity which can be used to solve the cubic equation.

The recurrence relation (n-1)A_(n+1)=(n^2-1)A_n+(n+1)A_(n-1)+4(-1)^n valid for n=4, 5, ... with A(2)=0 and A(3)=1 and which solves the married couples problem (Dörrie 1965, ...