A036524 Smallest triangular number containing exactly n 7's.

78, 1770, 72771, 6787770, 37779778, 2476777771, 7477727778, 7077757275771, 27778877747778, 177775772774775, 7777755777787716, 172777077787777785, 7987777777779277726, 177577777275777787371, 377679777777377717778, 77773137747757777775775, 777367777737777774977796, 67775787670776777773777778

Offset: 1

Author

David W. Wilson

Keywords

• nonn
• base

Crossrefs

Cf. A048362, A036517-A036526.

Programs

Mathematica

nsmall = Table[Infinity, 20];
For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i];
  n0 = Count[IntegerDigits[p], 7];
  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