A122853 Numbers k such that (3^k + 5^k)/8 = A074606(k)/8 is a prime.

3, 5, 7, 17, 19, 109, 509, 661, 709, 1231, 12889, 13043, 26723, 43963, 44789

Offset: 1

Author

Alexander Adamchuk, Sep 14 2006

Keywords

• nonn
• more

Comments

(3^k + 5^k)/8 = A074606(k)/8 = A081186(k)/4.

Corresponding primes of the form (3^k + 5^k)/2^3 are listed in {A121938(n)} = {A079773(a(n))} = {19, 421, 10039, 95383574161, 2384331073699, ...}.

No other terms less than 100000. - Robert Price, Apr 28 2012

Crossrefs

Cf. A074606, A081186, A121824, A121877, A005058, A005059, A121938, A109347, A079773.

Programs

Mathematica

Do[f=5^n+3^n;If[PrimeQ[f/2^3],Print[{n,f/2^3}]],{n,1,1231}]

PARI

select(n->isprime((3^n + 5^n)/8), vector(2000,i,i)) \\ Charles R Greathouse IV, Feb 13 2011

Extensions

a(11)-a(15) from Robert Price, Apr 28 2012