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A225440 Triangular numbers that are the product of three distinct triangular numbers greater than 1. 2
378, 630, 990, 3240, 4095, 4950, 5460, 9180, 15400, 19110, 25200, 31878, 37128, 37950, 39060, 52650, 61425, 79800, 97020, 103740, 105570, 122265, 145530, 157080, 161028, 176715, 192510, 221445, 265356, 288420, 304590, 306936, 346528, 437580, 500500, 545490, 583740 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Triangular numbers of the form triangular(x) * triangular(y) * triangular(z), x > y > z > 1.
LINKS
EXAMPLE
378 = 3 * 6 * 21.
630 = 3 * 10 * 21.
990 = 3 * 6 * 55.
PROG
(C)
#include <stdio.h>
typedef unsigned long long U64;
U64 isTriangular(U64 a) { // ! Must be a < (1<<63)
U64 s = sqrt(a*2);
if (a>=(1ULL<<63)) exit(1);
return (s*(s+1)/2 == a);
}
int compare64(const void *p1, const void *p2) {
if (*(U64*)p1 == *(U64*)p2) return 0;
if (*(U64*)p1 < *(U64*)p2) return -1;
return 1;
}
#define TOP (1<<21)
U64 d[TOP];
int main() {
U64 c, x, tx, y, ty, z, tz, p = 0;
for (x = tx = 3; tx <= TOP; tx+=x, ++x) {
for (y = ty = 3; ty < tx; ty+=y, ++y) {
for (z = tz = 3; tz < ty; tz+=z, ++z) {
c = tx*ty*tz;
if (c <= TOP*18 && isTriangular(c)) d[p++] = c;
}}}
qsort(d, p, 8, compare64);
for (x=c=0; c<p; ++c) if ((y=d[c])>x) printf("%llu, ", y), x=y;
return 0;
}
CROSSREFS
Sequence in context: A030029 A064242 A116339 * A098840 A065702 A132648
KEYWORD
nonn
AUTHOR
Alex Ratushnyak, May 08 2013
STATUS
approved

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Last modified May 30 17:56 EDT 2024. Contains 372971 sequences. (Running on oeis4.)