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%I #7 May 09 2021 06:07:22
%S 2,2,2,3,2,2,3,4,2,3,2,2,5,4,2,3,2,2,6,4,2,3,5,2,3,7,2,3,2,2,3,4,5,8,
%T 2,2,3,4,2,3,2,2,9,4,2,3,7,2,6,4,2,3,10,2,3,4,2,3,2,2,3,8,5,11,2,2,3,
%U 7,2,3,2,2,5,4,2,12,2,2,9,4,2,3,5,2,3,4,2,3,13,8,3,4,5,6,2,2,3,10,2,3,2,2
%N Index of n as m-gonal number for the smallest possible m (=A176774(n)).
%C a(n)=2 iff A176774(n)=n.
%H Michel Marcus, <a href="/A176775/b176775.txt">Table of n, a(n) for n = 3..10000</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/PolygonalNumber.html">Polygonal Number</a>.
%F a(n) = (m-4+sqrt((m-4)^2+8*(m-2)*n)) / (2*m-4), where m = A176774(n).
%o (PARI) f(n) = {k=3; while (! ispolygonal(n, k), k++); k; } \\ A176774
%o a(n) = my(m=f(n)); (m-4+sqrtint((m-4)^2+8*(m-2)*n)) / (2*m-4); \\ _Michel Marcus_, May 09 2021
%Y Cf. A176774.
%K nonn
%O 3,1
%A _Max Alekseyev_, Apr 25 2010