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A Lambert series is a series of the form F(x)=sum_(n=1)^inftya_n(x^n)/(1-x^n) (1) for |x|<1. Then F(x) = sum_(n=1)^(infty)a_nsum_(m=1)^(infty)x^(mn) (2) = ...

A dozen dozen, also called a gross. 144 is a square number and a sum-product number.

The portion of number theory concerned with expressing an integer as a sum of integers from some given set.

The difference between the sum of face angles A_i at a polyhedron vertex of a polyhedron and 2pi, delta=2pi-sum_(i)A_i.

A simple root of a Lie algebra is a positive root that is not the sum of two positive roots.

The volume of a prismatoid is equal to the sum of the volumes of a pyramid, a wedge, and a parallelepiped.

The divisibility test that an integer is divisible by 9 iff the sum of its digits is divisible by 9.

The sum of the angles of a triangle is two right angles. This postulate is equivalent to the parallel postulate.

A number n is called a k e-perfect number if sigma_e(n)=kn, where sigma_e(n) is the sum of the e-divisors of n.

A generalization of the Gaussian sum. For p and q of opposite parity (i.e., one is even and the other is odd), Schaar's identity states ...