OFFSET
2,1
COMMENTS
Conjecture: For any n > 1 we have a(n) < F(n+4); moreover, there are infinitely many integers b > F(n) such that sum_{k=1}^n F(k)*b^{k-1} is prime.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 2..49
EXAMPLE
a(6) = 10 since sum_{k=0}^6 F(k)*10^{k-1} = 853211 is prime but sum_{k=0}^6 F(k)*9^{k-1} = 507556 is composite.
MATHEMATICA
A[n_, x_]:=A[n, x]=Sum[Fibonacci[k]*x^(k-1), {k, 1, n}]
Do[Do[Do[If[PrimeQ[A[n, s]]==True, Print[n, " ", s]; Goto[aa]], {s, Fibonacci[n]+1, Fibonacci[n+4]-1}];
Print[n, " ", counterexample]; Label[aa]; Continue, {n, 2, 20}]]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Apr 08 2013
STATUS
approved