A205847 - OEIS

%I #9 Oct 25 2024 04:53:14

%S 4,6,6,7,7,7,8,9,10,10,10,10,11,12,12,12,12,12,13,13,13,13,13,13,14,

%T 14,15,15,16,16,16,16,16,16,16,17,17,18,18,18,18,18,18,18,18,19,19,19,

%U 19,19,19,19,19,19,20,20,20,21,21,21

%N Numbers k for which 4 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(4)-s(1) = 5-1 = 4

%e s(6)-s(1) = 13-1 = 12

%e s(6)-s(4) = 13-5 = 8

%e s(7)-s(1) = 21-1 = 20

%e s(7)-s(4) = 21-5 = 16

%e s(7)-s(6) = 21-13 = 8

%t s[n_] := s[n] = Fibonacci[n + 1]; z1 = 400; z2 = 60;

%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 = 4; t = d[c]    (* A205846 *)

%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}]    (* A205847 *)

%t Table[j[n], {n, 1, z2}]    (* A205848 *)

%t Table[s[k[n]] - s[j[n]], {n, 1, z2}] (* A205849 *)

%t Table[(s[k[n]] - s[j[n]])/c, {n, 1, z2}] (* A205850 *)

%Y Cf. A204892, A205850.

%K nonn

%O 1,1

%A _Clark Kimberling_, Feb 02 2012