heptagonal numbers

pl. Arith.: A kind of polygonal numbers, formed as follows: Let 1, 6, 11, 16, 21, 26, ., be arithmetical progre ion, then 1, 7, 18, 34, 55, 81, ., will be the series of polygonal numbers, called heptagonal. The law of formation is to add each number in the lower line to the next one on the right in the upper line. The general formula for heptagonal numbers is 5n2-3n N = IMG:content-1120.png:[blocks in formation] hěp-ta-gyn, s. [HEPTAGYNIA.] Bot.: A plant of the Linnæan cla Heptagynia (q. v.).