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
Links
- Eric Weisstein's World of Mathematics, Triangular Number
Crossrefs
Programs
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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 *)
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