A048356 a(n) is the index of the smallest triangular number containing exactly n 1's.

1, 18, 58, 458, 1541, 6018, 47141, 149406, 471404, 4756366, 15563541, 149075893, 149004101, 149071198, 14907133333, 47322533981, 471406654834, 1510636363333, 4714049577658, 17954448535733, 471151591552466, 1490649602094141, 4519177339165085, 15434448560354601, 47140495990478741

Offset

1,2

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], 1];
  If[nsmall[[n0]] > i, nsmall[[n0]] = i]];
ReplaceAll[nsmall, Infinity -> "?"] (* Robert Price, Mar 22 2020 *)

Crossrefs

Cf. A036518, A048346, A048355, A048357, A048358, A048359, A048360, A048361, A048362, A048363, A048364, A048531.

Sequence in context: A298765 A299460 A155155 * A244806 A027055 A056448

Adjacent sequences: A048353 A048354 A048355 * A048357 A048358 A048359

Keyword

nonn,base,changed

Author

Patrick De Geest, Mar 15 1999

Extensions

a(15)-a(16) from Lars Blomberg, May 13 2011

a(17)-a(18) from Giovanni Resta, Oct 30 2019

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

Status

approved