OFFSET
1,3
COMMENTS
Conjecture: a(n) > 0 for all n > 1.
See also the conjecture in A234388.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..4000
EXAMPLE
a(7) = 2 since 2^1*(2^{phi(6)-1) + 1 = 2*3 + 1 = 7 and 2^2*(2^{phi(5)}-1) + 1 = 4*15 + 1 = 61 are both prime.
MATHEMATICA
f[n_, k_]:=f[n, k]=2^k*(2^(EulerPhi[n-k])-1)+1
a[n_]:=Sum[If[PrimeQ[f[n, k]], 1, 0], {k, 1, n-1}]
Table[a[n], {n, 1, 100}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Dec 25 2013
STATUS
approved