A048360 a(n) is the index of the smallest triangular number containing exactly n 5's.
5, 10, 105, 714, 5010, 34221, 103494, 317885, 3243010, 1054130, 55708989, 280555005, 1051237885, 5206833885, 30184681729, 105409283410, 1053999578610, 1005540238434, 8894447217885, 178132285431643, 717642746162114, 3273082806882696, 10445626412576275, 33332433321191885, 105428230332824189
Offset: 1
Links
- Eric Weisstein's World of Mathematics, Triangular Number
Crossrefs
Programs
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Mathematica
nsmall = Table[Infinity, 20]; For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i]; n0 = Count[IntegerDigits[p], 5]; If[nsmall[[n0]] > i, nsmall[[n0]] = i]]; ReplaceAll[nsmall, Infinity -> "?"] (* Robert Price, Mar 22 2020 *)
Extensions
a(13)-a(15) from Lars Blomberg, May 16 2011
a(16)-a(18) from Giovanni Resta, Oct 30 2019
a(19)-a(25) from Max Alekseyev, Mar 07 2025