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

1,1

Links

Table of n, a(n) for n=1..25.

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], 5];
  If[nsmall[[n0]] > i, nsmall[[n0]] = i]];
ReplaceAll[nsmall, Infinity -> "?"] (* Robert Price, Mar 22 2020 *)

Crossrefs

Cf. A036522, A048350, A048355, A048356, A048357, A048358, A048359, A048361, A048362, A048363, A048364, A048539.

Sequence in context: A344140 A267042 A119137 * A357565 A251702 A067958

Adjacent sequences: A048357 A048358 A048359 * A048361 A048362 A048363

Keyword

nonn,base,changed

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

a(19)-a(25) from Max Alekseyev, Mar 07 2025

Status

approved