cp's OEIS Frontend

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

%I A225400 #9 May 07 2013 13:31:15
%S A225400 0,3,8,14,15,39,20,44,35,195,119,104,594,224,384,455,539,440,560,3080,
%T A225400 2184,1539,2015,2639,5264,4199,15399,13299,8855,23919,2079,30744,
%U A225400 43680,36575,14399,5984,58695,113399,47124,107184,12375,78624,98175,73359,111320,242879
%N A225400 First occurrence of n in A225399, or -1 if n does not appear in A225399.
%C A225400 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.
%C A225400 Conjecture: a(n) >= 0.
%F A225400 A000217(a(n)) = A076983(n+1) for n>0, if the conjecture is true and a(n) >= 0.
%t A225400 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 *)
%o A225400 (C)
%o A225400 #include <stdio.h>
%o A225400 #define TOP 80
%o A225400 int main() {
%o A225400   unsigned long long c, i, j, t, tn;
%o A225400   long long f[TOP];
%o A225400   memset(f, -1, sizeof(f));
%o A225400   for (i = tn = 0; i < (1ULL<<32); i++) {
%o A225400         for (c=0, tn += i, t = j = 3; t*2 <= tn; t+=j, ++j)
%o A225400                 if (tn%t==0) ++c;
%o A225400         if (c<TOP && f[c]<0)
%o A225400                 for (printf("\n"), f[c]=i, j=0; j<TOP; ++j)
%o A225400                         printf("%lld, ", f[j]);
%o A225400   }
%o A225400   return 0;
%o A225400 }
%Y A225400 Cf. A000217, A225399, A076983.
%K A225400 nonn
%O A225400 0,2
%A A225400 _Alex Ratushnyak_, May 06 2013