A048359 - OEIS

%I #16 Mar 22 2020 23:42:32

%S 9,94,537,1043,29238,28790,94238,942703,2967970,45748113,170565790,

%T 939618480,2981759361,29814031731,94285146809,298802358238,

%U 2837045748113,9428516473384

%N a(n) is the index of the smallest triangular number containing exactly n 4'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], 4];

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

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

%Y Cf. A036521, A048355-A049364.

%K nonn,base,more

%O 1,1

%A _Patrick De Geest_, Mar 15 1999

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

%E a(15)-a(18) from _Giovanni Resta_, Oct 30 2019