Heptagonal numbers

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Related to Heptagonal numbers: Triangular numbers, Square numbers, Pentagonal numbers
(Arith.) the numbers of the series 1, 7, 18, 34, 55, etc., being figurate numbers formed by adding successively the terms of the arithmetical series 1, 6, 11, 16, 21, etc.

See also: Heptagonal

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive ?
After this they should go on to establish the hexagonal numbers HX(n) and the heptagonal numbers HP(n).