OFFSET
0,9
LINKS
Eric Weisstein's World of Mathematics, Heptagonal Number
EXAMPLE
a(26) = 6 because we have [18, 7, 1], [18, 1, 7], [7, 18, 1], [7, 1, 18], [1, 18, 7] and [1, 7, 18].
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Feb 04 2020
STATUS
approved