Stella octangula number
In mathematics, a stella octangula number is a figurate number based on the stella octangula, of the form .

The sequence of stella octangula numbers is
0, 1, 14, 51, 124, 245, 426, 679, 1016, 1449, 1990, ... 

Only two of these numbers are square.

Ljunggren's equation
There are only two positive square stella octangula numbers,  and , corresponding to  and  respectively. The elliptic curve describing the square stella octangula numbers,

may be placed in the equivalent Weierstrass form

by the change of variables , . Because the two factors  and  of the square number  are relatively prime, they must each be squares themselves, and the second change of variables  and  leads to Ljunggren's equation

A theorem of Siegel states that every elliptic curve has only finitely many integer solutions, and  found a difficult proof that the only integer solutions to his equation were  and , corresponding to the two square stella octangula numbers. Louis J. Mordell conjectured that the proof could be simplified, and several later authors published simplifications.

Additional applications
The stella octangula numbers arise in a parametric family of instances to the crossed ladders problem in which the lengths and heights of the ladders and the height of their crossing point are all integers. In these instances, the ratio between the heights of the two ladders is a stella octangula number.

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Figurate numbers