A048362 a(n) is the index of the smallest triangular number containing exactly n 7's.
12, 59, 381, 3684, 8692, 70381, 122292, 3762381, 7453707, 18856074, 124721736, 587838545, 3996943276, 18845571218, 27483805332, 394393554074, 1246890354231, 11642661866667, 88292437929618, 184774336842416, 1158255177391940, 5962836170393707, 37870196138329618, 59623111421253050, 107479087154457892
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], 7]; 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