A205872 - OEIS

%I #6 Mar 30 2012 18:58:12

%S 7,9,10,12,12,13,13,13,14,16,17,17,18,19,20,21,21,21,21,22,22,22,23,

%T 24,24,24,24,25,25,25,25,25,26,26,27,27,28,28,29,29,29,29,29,30,30,31,

%U 31,31,32,32

%N Numbers k for which 9 divides s(k)-s(j) for some j<k; each k occurs once for each such j; s(k) denotes the (k+1)-st Fibonacci number.

%C For a guide to related sequences, see A205840.

%e The first six terms match these differences:

%e s(7)-s(3) = 21-3 = 18 = 9*2

%e s(9)-s(1) = 55-1 = 54 = 9*6

%e s(10)-s(5) = 89-8 = 81 = 9*9

%e s(12)-s(5) = 233-8 = 225 = 9*25

%e s(12)-s(10) = 233-89 = 144 = 9*16

%e s(13)-s(5) = 377-8 = 369 =9*41

%t s[n_] := s[n] = Fibonacci[n + 1]; z1 = 600; z2 = 50;

%t f[n_] := f[n] = Floor[(-1 + Sqrt[8 n - 7])/2];

%t Table[s[n], {n, 1, 30}]

%t u[m_] := u[m] = Flatten[Table[s[k] - s[j], {k, 2, z1}, {j, 1, k - 1}]][[m]]

%t Table[u[m], {m, 1, z1}]   (* A204922 *)

%t v[n_, h_] := v[n, h] = If[IntegerQ[u[h]/n], h, 0]

%t w[n_] := w[n] = Table[v[n, h], {h, 1, z1}]

%t d[n_] := d[n] = Delete[w[n], Position[w[n], 0]]

%t c = 9; t = d[c]     (* A205871 *)

%t k[n_] := k[n] = Floor[(3 + Sqrt[8 t[[n]] - 1])/2]

%t j[n_] := j[n] = t[[n]] - f[t][[n]] (f[t[[n]]] + 1)/2

%t Table[k[n], {n, 1, z2}]       (* A205872 *)

%t Table[j[n], {n, 1, z2}]         (* A205873 *)

%t Table[s[k[n]] - s[j[n]], {n, 1, z2}]   (* A205874 *)

%t Table[(s[k[n]] - s[j[n]])/c, {n, 1, z2}]  (* A205875 *)

%Y Cf. A204892, A205873, A205875.

%K nonn

%O 1,1

%A _Clark Kimberling_, Feb 02 2012