cp's OEIS Frontend

A225884 Triangular numbers whose binary and decimal reversals are also triangular numbers.

%I A225884 #27 Jan 11 2016 03:55:17
%S A225884 0,1,3,6,120,153,300
%N A225884 Triangular numbers whose binary and decimal reversals are also triangular numbers.
%C A225884 A subsequence of A061455.
%C A225884 a(8), if it exists, is > triangular(10^11) > 5*10^21. - _Lars Blomberg_, Jan 11 2016
%e A225884 BinaryReverse(120) = 15, DecimalReverse(120) = 21. Because 120, 15 and 21 are triangular numbers, 120 is in the sequence.
%o A225884 (C)
%o A225884 #include <stdio.h>
%o A225884 #include <stdlib.h>
%o A225884 #include <math.h>
%o A225884 int isTriangular(unsigned long long a) {
%o A225884     unsigned long long sr = sqrt(a*2);
%o A225884     return (sr*(sr+1) == a*2);
%o A225884 }
%o A225884 int main() {
%o A225884   unsigned long long n, tn, t, r;
%o A225884   for (n = tn = 0; tn < (1ULL<<63); tn += ++n) {
%o A225884     for (r=0, t=tn; t; t>>=1)  r = r*2 + (t&1);
%o A225884     if (isTriangular(r)==0) continue;
%o A225884     for (r=0, t=tn; t; t/=10)  r = r*10 + (t%10);
%o A225884     if (isTriangular(r)==0) continue;
%o A225884     printf("%llu, ", tn);
%o A225884   }
%o A225884   return 0;
%o A225884 }
%Y A225884 Cf. A000217, A061455.
%K A225884 nonn,base,more
%O A225884 1,3
%A A225884 _Alex Ratushnyak_, May 24 2013