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%I #17 Feb 24 2024 01:15:55
%S 1,5,19,25,199,625,1999,2102,5582,9376,9909,10100,16833,19999,41012,
%T 55741,74491,83885,88102,90625,151774,199999,201002,558257,626522,
%U 740312,798097,810062,890625,1001000,1030563,1668333,1999999,2646751
%N Numbers k such that the decimal representation of k is contained as substring in that of the k-th triangular number.
%H Giovanni Resta, <a href="/A119238/b119238.txt">Table of n, a(n) for n = 1..101</a> (terms < 10^12)
%t Select[Range@100000, ({}!=StringPosition[ToString[ #(#+1)/2], ToString@# ]) &]
%t Select[Range[3*10^6],SequenceCount[IntegerDigits[(#(#+1))/2], IntegerDigits[ #]]>0&] (* The program uses the SequenceCount function from Mathematica version 10 *) (* _Harvey P. Dale_, Dec 26 2015 *)
%Y Cf. A046831.
%K base,nonn
%O 1,2
%A _Giovanni Resta_, May 10 2006
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