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A176775 Index of n as m-gonal number for the smallest possible m (=A176774(n)). 9
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, 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, 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 (list; graph; refs; listen; history; text; internal format)
OFFSET
3,1
COMMENTS
a(n)=2 iff A176774(n)=n.
LINKS
Eric Weisstein's World of Mathematics, Polygonal Number.
FORMULA
a(n) = (m-4+sqrt((m-4)^2+8*(m-2)*n)) / (2*m-4), where m = A176774(n).
PROG
(PARI) f(n) = {k=3; while (! ispolygonal(n, k), k++); k; } \\ A176774
a(n) = my(m=f(n)); (m-4+sqrtint((m-4)^2+8*(m-2)*n)) / (2*m-4); \\ Michel Marcus, May 09 2021
CROSSREFS
Cf. A176774.
Sequence in context: A226056 A104011 A242879 * A175778 A357039 A226182
KEYWORD
nonn
AUTHOR
Max Alekseyev, Apr 25 2010
STATUS
approved

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Last modified June 17 06:37 EDT 2024. Contains 373432 sequences. (Running on oeis4.)