A225400 First occurrence of n in A225399, or -1 if n does not appear in A225399.

0, 3, 8, 14, 15, 39, 20, 44, 35, 195, 119, 104, 594, 224, 384, 455, 539, 440, 560, 3080, 2184, 1539, 2015, 2639, 5264, 4199, 15399, 13299, 8855, 23919, 2079, 30744, 43680, 36575, 14399, 5984, 58695, 113399, 47124, 107184, 12375, 78624, 98175, 73359, 111320, 242879

Offset

0,2

Comments

Index of the least triangular number t divisible by exactly n triangular numbers bigger than 1 and less than t, or -1 if there is no such t.

Conjecture: a(n) >= 0.

Links

Table of n, a(n) for n=0..45.

Formula

A000217(a(n)) = A076983(n+1) for n>0, if the conjecture is true and a(n) >= 0.

Mathematica

mx = 10000; tri = Table[n (n + 1)/2, {n, mx}]; nn = 20; t = Table[0, {nn}]; found = 0; n = 0; While[n < mx && found < nn, n++; cnt = 0; Do[If[Mod[tri[[n]], tri[[k]]] == 0, cnt++], {k, 2, n - 1}]; If[cnt <= nn && t[[cnt]] == 0, t[[cnt]] = n; found++]]; Join[{0}, t] (* T. D. Noe, May 07 2013 *)

Prog

(C)
#include <stdio.h>
#define TOP 80
int main() {
  unsigned long long c, i, j, t, tn;
  long long f[TOP];
  memset(f, -1, sizeof(f));
  for (i = tn = 0; i < (1ULL<<32); i++) {
        for (c=0, tn += i, t = j = 3; t*2 <= tn; t+=j, ++j)
                if (tn%t==0) ++c;
        if (c<TOP && f[c]<0)
                for (printf("\n"), f[c]=i, j=0; j<TOP; ++j)
                        printf("%lld, ", f[j]);
  }
  return 0;
}

Crossrefs

Cf. A000217, A225399, A076983.

Sequence in context: A375292 A361363 A067789 * A180867 A343412 A063218

Adjacent sequences: A225397 A225398 A225399 * A225401 A225402 A225403

Keyword

nonn

Author

Alex Ratushnyak, May 06 2013

Status

approved