cp's OEIS Frontend

A225440 Triangular numbers that are the product of three distinct triangular numbers greater than 1.

%I A225440 #6 May 08 2013 11:45:57
%S A225440 378,630,990,3240,4095,4950,5460,9180,15400,19110,25200,31878,37128,
%T A225440 37950,39060,52650,61425,79800,97020,103740,105570,122265,145530,
%U A225440 157080,161028,176715,192510,221445,265356,288420,304590,306936,346528,437580,500500,545490,583740
%N A225440 Triangular numbers that are the product of three distinct triangular numbers greater than 1.
%C A225440 Triangular numbers of the form triangular(x) * triangular(y) * triangular(z), x > y > z > 1.
%H A225440 Donovan Johnson, <a href="/A225440/b225440.txt">Table of n, a(n) for n = 1..1000</a>
%e A225440 378 = 3 * 6 * 21.
%e A225440 630 = 3 * 10 * 21.
%e A225440 990 = 3 * 6 * 55.
%o A225440 (C)
%o A225440 #include <stdio.h>
%o A225440 typedef unsigned long long U64;
%o A225440 U64 isTriangular(U64 a) {  // ! Must be a < (1<<63)
%o A225440     U64 s = sqrt(a*2);
%o A225440     if (a>=(1ULL<<63)) exit(1);
%o A225440     return (s*(s+1)/2 == a);
%o A225440 }
%o A225440 int compare64(const void *p1, const void *p2) {
%o A225440   if (*(U64*)p1 == *(U64*)p2) return 0;
%o A225440   if (*(U64*)p1 < *(U64*)p2) return -1;
%o A225440   return 1;
%o A225440 }
%o A225440 #define TOP (1<<21)
%o A225440 U64 d[TOP];
%o A225440 int main() {
%o A225440   U64 c, x, tx, y, ty, z, tz, p = 0;
%o A225440     for (x = tx = 3; tx <= TOP; tx+=x, ++x) {
%o A225440     for (y = ty = 3; ty < tx;   ty+=y, ++y) {
%o A225440     for (z = tz = 3; tz < ty;   tz+=z, ++z) {
%o A225440     c = tx*ty*tz;
%o A225440     if (c <= TOP*18 && isTriangular(c))  d[p++] = c;
%o A225440   }}}
%o A225440   qsort(d, p, 8, compare64);
%o A225440   for (x=c=0; c<p; ++c) if ((y=d[c])>x) printf("%llu, ", y), x=y;
%o A225440   return 0;
%o A225440 }
%Y A225440 Cf. A000217, A085780, A140089, A188630, A225390.
%K A225440 nonn
%O A225440 1,1
%A A225440 _Alex Ratushnyak_, May 08 2013