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From: Webster's Revised Unabridged Dictionary (1913)

 Hep·tag·o·nal a.  Having seven angles or sides.
 Heptagonal 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.
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