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%I #27 Jan 11 2016 03:55:17
%S 0,1,3,6,120,153,300
%N Triangular numbers whose binary and decimal reversals are also triangular numbers.
%C A subsequence of A061455.
%C a(8), if it exists, is > triangular(10^11) > 5*10^21. - _Lars Blomberg_, Jan 11 2016
%e BinaryReverse(120) = 15, DecimalReverse(120) = 21. Because 120, 15 and 21 are triangular numbers, 120 is in the sequence.
%o (C)
%o #include <stdio.h>
%o #include <stdlib.h>
%o #include <math.h>
%o int isTriangular(unsigned long long a) {
%o unsigned long long sr = sqrt(a*2);
%o return (sr*(sr+1) == a*2);
%o }
%o int main() {
%o unsigned long long n, tn, t, r;
%o for (n = tn = 0; tn < (1ULL<<63); tn += ++n) {
%o for (r=0, t=tn; t; t>>=1) r = r*2 + (t&1);
%o if (isTriangular(r)==0) continue;
%o for (r=0, t=tn; t; t/=10) r = r*10 + (t%10);
%o if (isTriangular(r)==0) continue;
%o printf("%llu, ", tn);
%o }
%o return 0;
%o }
%Y Cf. A000217, A061455.
%K nonn,base,more
%O 1,3
%A _Alex Ratushnyak_, May 24 2013
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