A figurate number corresponding to a pentagonal pyramid. The first few are 1, 6, 18, 40, 75, ... (Sloane's A002411). The generating function for the pentagonal pyramidal numbers is

The odd pentagonal pyramidal numbers are given by 1, 75, 405, 1183, 2601, ... (Sloane's A015223), having squares 1, 5625, 164025, ... (Sloane's A014799), while the even pentagonal pyramidal numbers are given by 6, 18, 40, 126, 196, 288, ... (Sloane's A015224), having squares 36, 324, 1600, 15876, ... (Sloane's A014800).

Sloane, N. J. A. Sequences A002411/M4116, A014799, A014800, A015223, and A015224 in "The On-Line Encyclopedia of Integer Sequences."

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Weisstein, Eric W. "Pentagonal Pyramidal Number." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/PentagonalPyramidalNumber.html