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

Offset

1,1

Links

Table of n, a(n) for n=1..18.

Eric Weisstein's World of Mathematics, Triangular Number

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 *)

Crossrefs

Cf. A036524, A048355-A049364.

Sequence in context: A167533 A244907 A062477 * A231215 A231340 A099321

Adjacent sequences: A048359 A048360 A048361 * A048363 A048364 A048365

Keyword

nonn,base,more

Author

Patrick De Geest, Mar 15 1999

Extensions

a(13)-a(15) from Lars Blomberg, May 16 2011

a(16)-a(18) from Giovanni Resta, Oct 30 2019

Status

approved