A048364 a(n) is the index of the smallest triangular number containing exactly n 9's.

13, 44, 631, 1413, 28282, 76419, 282842, 1414213, 4471955, 31616419, 446313656, 1414001413, 6276925986, 44710913656, 44721356419, 632129732586, 527257052109, 14141923468538

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], 9];
  If[nsmall[[n0]] > i, nsmall[[n0]] = i]];
ReplaceAll[nsmall, Infinity -> "?"] (* Robert Price, Mar 22 2020 *)

Crossrefs

Cf. A036526, A048355-A049363.

Sequence in context: A226514 A159742 A098385 * A254675 A141549 A121964

Adjacent sequences: A048361 A048362 A048363 * A048365 A048366 A048367

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