A036522 Smallest triangular number containing exactly n 5's.

15, 55, 5565, 255255, 12552555, 585555531, 5355555765, 50525595555, 5258558551555, 555595555515, 1551745755555555, 39355555555552515, 552550545955255555, 13555559555595513555, 455557505555605555585, 5555558514557555255755, 555457555855555784555355, 505555585555455555505395

Offset: 1

Author

David W. Wilson

Keywords

• nonn
• base

Crossrefs

Cf. A048360, A036517-A036526.

Programs

Mathematica

nsmall = Table[Infinity, 20];
For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i];
  n0 = Count[IntegerDigits[p], 5];
  If[nsmall[[n0]] > p, nsmall[[n0]] = p]];
ReplaceAll[nsmall, Infinity -> "?"] (* Robert Price, Mar 22 2020 *)

Extensions

a(13)-a(18) from Giovanni Resta, Oct 30 2019