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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 Donovan Johnson, Table of n, a(n) for n = 1..1000 EXAMPLE 378 = 3 * 6 * 21. 630 = 3 * 10 * 21. 990 = 3 * 6 * 55. PROG (C) #include 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; cx) printf("%llu, ", y), x=y; return 0; } CROSSREFS Cf. A000217, A085780, A140089, A188630, A225390. Sequence in context: A030029 A064242 A116339 * A098840 A065702 A132648 Adjacent sequences: A225437 A225438 A225439 * A225441 A225442 A225443 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.)