A226150 Smallest of three consecutive primes whose average is a triangular number.

18713, 27253, 35227, 45433, 138587, 251677, 283861, 425489, 462221, 463189, 486583, 634493, 694409, 826211, 943231, 1103341, 1163557, 1181927, 1214453, 1282387, 1462891, 1509439, 1925681, 1931569, 2425487, 2970689, 3041803, 3324323, 3605939, 3627451, 4096933, 5140781

Offset

1,1

Links

Table of n, a(n) for n=1..32.

Prog

(C)
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define TOP (1ULL<<30)
int main() {
  unsigned long long i, j, p1, p2, r, s;
  unsigned char *c = (unsigned char *)malloc(TOP/8);
  memset(c, 0, TOP/8);
  for (i=3; i < TOP; i+=2)
    if ((c[i>>4] & (1<<((i>>1) & 7)))==0 /*&& i<(1ULL<<32)*/)
        for (j=i*i>>1; j<TOP; j+=i)  c[j>>3] |= 1 << (j&7);
  for (p2=2, p1=3, i=5; i < TOP; i+=2)
    if ((c[i>>4] & (1<<((i>>1) & 7)))==0) {
      s = p2 + p1 + i;
      if ((s%3)==0) {
        s/=3;
        r = sqrt(s*2);
        if (r*(r+1)==s*2) printf("%llu, ", p2);
      }
      p2 = p1, p1 = i;
    }
  return 0;
}

Crossrefs

Cf. A076304, A206279, A226145-A226149.

Sequence in context: A283934 A266061 A267028 * A081416 A051795 A338949

Adjacent sequences: A226147 A226148 A226149 * A226151 A226152 A226153

Keyword

nonn

Author

Alex Ratushnyak, May 28 2013

Status

approved