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

0, 1, 3, 6, 120, 153, 300

Offset: 1

Author

Alex Ratushnyak, May 24 2013

Keywords

• nonn
• base
• more

Comments

A subsequence of A061455.

a(8), if it exists, is > triangular(10^11) > 5*10^21. - Lars Blomberg, Jan 11 2016

Examples

			BinaryReverse(120) = 15, DecimalReverse(120) = 21. Because 120, 15 and 21 are triangular numbers, 120 is in the sequence.
		

Crossrefs

Cf. A000217, A061455.

Programs

C

#include 
#include 
#include 
int isTriangular(unsigned long long a) {
    unsigned long long sr = sqrt(a*2);
    return (sr*(sr+1) == a*2);
}
int main() {
  unsigned long long n, tn, t, r;
  for (n = tn = 0; tn < (1ULL<<63); tn += ++n) {
    for (r=0, t=tn; t; t>>=1)  r = r*2 + (t&1);
    if (isTriangular(r)==0) continue;
    for (r=0, t=tn; t; t/=10)  r = r*10 + (t%10);
    if (isTriangular(r)==0) continue;
    printf("%llu, ", tn);
  }
  return 0;
}