

A205857


Numbers k for which 6 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.


6



5, 6, 7, 9, 9, 10, 12, 12, 13, 13, 13, 14, 15, 15, 16, 16, 16, 17, 17, 18, 18, 18, 18, 19, 19, 19, 20, 20, 21, 21, 21, 21, 21, 22, 22, 22, 22, 23, 24, 24, 24, 24, 24, 25, 25, 25, 25, 25, 25, 26, 26, 26, 27, 27, 27, 27, 28, 28, 28, 28
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OFFSET

1,1


COMMENTS

For a guide to related sequences, see A205840.


LINKS

Table of n, a(n) for n=1..60.


EXAMPLE

The first six terms match these differences:
s(5)s(2) = 82 = 6 = 6*1
s(6)s(1) = 131 = 12 = 6*2
s(7)s(3) = 213 = 18 = 6*3
s(9)s(1) = 551 = 54 = 6*9
s(9)s(6) = 5513 = 42 = 6*7
s(10)s(4) = 895 = 84 =6*14


MATHEMATICA

s[n_] := s[n] = Fibonacci[n + 1]; z1 = 500; z2 = 60;
f[n_] := f[n] = Floor[(1 + Sqrt[8 n  7])/2];
Table[s[n], {n, 1, 30}]
u[m_] := u[m] = Flatten[Table[s[k]  s[j], {k, 2, z1}, {j, 1, k  1}]][[m]]
Table[u[m], {m, 1, z1}] (* A204922 *)
v[n_, h_] := v[n, h] = If[IntegerQ[u[h]/n], h, 0]
w[n_] := w[n] = Table[v[n, h], {h, 1, z1}]
d[n_] := d[n] = Delete[w[n], Position[w[n], 0]]
c = 6; t = d[c] (* A205856 *)
k[n_] := k[n] = Floor[(3 + Sqrt[8 t[[n]]  1])/2]
j[n_] := j[n] = t[[n]]  f[t][[n]] (f[t[[n]]] + 1)/2
Table[k[n], {n, 1, z2}] (* A205857 *)
Table[j[n], {n, 1, z2}] (* A205858 *)
Table[s[k[n]]s[j[n]], {n, 1, z2}] (* A205859 *)
Table[(s[k[n]]s[j[n]])/c, {n, 1, z2}] (* A205860 *)


CROSSREFS

Cf. A204892, A205857, A205860.
Sequence in context: A320021 A081407 A268857 * A196026 A191850 A066263
Adjacent sequences: A205854 A205855 A205856 * A205858 A205859 A205860


KEYWORD

nonn


AUTHOR

Clark Kimberling, Feb 02 2012


STATUS

approved



