A048357 - OEIS

%I #16 Mar 22 2020 23:42:19

%S 6,49,215,651,6651,66367,206020,635959,6651759,21411348,66651759,

%T 666816649,666651759,6666651759,64687340681,210818509825,

%U 1115546688777,6666665142651

%N a(n) is the index of the smallest triangular number containing exactly n 2's.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/TriangularNumber.html">Triangular Number</a>

%t nsmall = Table[Infinity, 20];

%t For[i = 0, i <= 10^6, i++, p = PolygonalNumber[i];

%t   n0 = Count[IntegerDigits[p], 2];

%t   If[nsmall[[n0]] > i, nsmall[[n0]] = i]];

%t ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *)

%Y Cf. A036519, A048355-A049364.

%K nonn,base,more

%O 1,1

%A _Patrick De Geest_, Mar 15 1999

%E a(14)-a(15) from _Lars Blomberg_, May 13 2011

%E a(16)-a(18) from _Giovanni Resta_, Oct 30 2019