A360663 a(n) is the least integer m >= 3 such that n is a centered m-gonal number.
3, 4, 5, 6, 7, 8, 3, 10, 11, 4, 13, 14, 5, 16, 17, 3, 19, 20, 7, 22, 23, 4, 25, 26, 9, 28, 29, 3, 31, 32, 11, 34, 35, 6, 37, 38, 13, 4, 41, 7, 43, 44, 3, 46, 47, 8, 49, 5, 17, 52, 53, 9, 55, 56, 19, 58, 59, 4, 61, 62, 3, 64, 65, 11, 67, 68, 23, 7, 71, 12, 73, 74, 5, 76, 77, 13, 79
Offset: 4
Keywords
Examples
a(16) = 5 since 16 is a centered pentagonal number, but not a centered square or centered triangular number.
Links
- Eric Weisstein's World of Mathematics, Centered Polygonal Number.
- Wikipedia, Centered polygonal number.
Programs
-
Mathematica
seq[len_] := Module[{s = Table[0, {len}], c = 0, k = 3, n, ckn}, While[c < len, n = 2; While[(ckn = k*n*(n - 1)/2 - 2) <= len, If[s[[ckn]] == 0, c++; s[[ckn]] = k]; n++]; n = 4; k++]; s]; seq[100] (* Amiram Eldar, Mar 06 2023 *)