A048357 a(n) is the index of the smallest triangular number containing exactly n 2's.

6, 49, 215, 651, 6651, 66367, 206020, 635959, 6651759, 21411348, 66651759, 666816649, 666651759, 6666651759, 64687340681, 210818509825, 1115546688777, 6666665142651

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], 2];
  If[nsmall[[n0]] > i, nsmall[[n0]] = i]];
ReplaceAll[nsmall, Infinity -> "?"] (* Robert Price, Mar 22 2020 *)

Crossrefs

Cf. A036519, A048355-A049364.

Sequence in context: A028308 A173011 A061429 * A027766 A097299 A283226

Adjacent sequences: A048354 A048355 A048356 * A048358 A048359 A048360

Keyword

nonn,base,more

Author

Patrick De Geest, Mar 15 1999

Extensions

a(14)-a(15) from Lars Blomberg, May 13 2011

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

Status

approved