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%I #16 Mar 22 2020 23:43:08
%S 13,44,631,1413,28282,76419,282842,1414213,4471955,31616419,446313656,
%T 1414001413,6276925986,44710913656,44721356419,632129732586,
%U 527257052109,14141923468538
%N a(n) is the index of the smallest triangular number containing exactly n 9'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], 9];
%t If[nsmall[[n0]] > i, nsmall[[n0]] = i]];
%t ReplaceAll[nsmall, Infinity -> "?"] (* _Robert Price_, Mar 22 2020 *)
%Y Cf. A036526, A048355-A049363.
%K nonn,base,more
%O 1,1
%A _Patrick De Geest_, Mar 15 1999
%E a(13)-a(15) from _Lars Blomberg_, May 16 2011
%E a(16)-a(18) from _Giovanni Resta_, Oct 30 2019