OFFSET
1,5
COMMENTS
Conjecture: a(n) > 0 for all n > 3.
See also the conjecture in A234337.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..3000
EXAMPLE
a(4) = 1 since 3^1 + 3^{phi(3)/2} - 1 = 5 is prime.
a(5) = 2 since 3^1 + 3^{phi(4)/2} - 1 = 5 and 3^2 + 3^{phi(3)/2} - 1 are both prime.
MATHEMATICA
f[n_, k_]:=3^k+3^(EulerPhi[n-k]/2)-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 24 2013
STATUS
approved