A048361 a(n) is the index of the smallest triangular number containing exactly n 6's.
3, 11, 36, 363, 3366, 8563, 35963, 394123, 3055048, 11531588, 115464831, 115470036, 1154484036, 10645763163, 35401884036, 152754484036, 365148371588, 2292012515963, 33065591380305, 115470003521836, 642442319075676, 1154701378423588, 10996978373724908, 36347397889274188, 230810167309875091
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], 6]; If[nsmall[[n0]] > i, nsmall[[n0]] = i]]; ReplaceAll[nsmall, Infinity -> "?"] (* Robert Price, Mar 22 2020 *)
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