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A048357 a(n) is the index of the smallest triangular number containing exactly n 2's. 1
6, 49, 215, 651, 6651, 66367, 206020, 635959, 6651759, 21411348, 66651759, 666816649, 666651759, 6666651759, 64687340681, 210818509825, 1115546688777, 6666665142651 (list; graph; refs; listen; history; text; internal format)
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

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Last modified September 23 20:42 EDT 2021. Contains 347617 sequences. (Running on oeis4.)