RE A007689: all terms composite?

[Tautócrona]

----- Original Message ----- 
From: "zak seidov" <zakseidov at yahoo.com>

For n=1,3,5,7,... a(n) are divisible by 5;
a(n) are primes for n=0,1,2,4.
My Q: what about other terms a(2k),
are they all composite?
Thanks, zak

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If we consider a(4k+2), then:

2^4 = = 3^4 = = 3 (mod 13)

2^(4k+2) + 3^(4k+2) = =
= = 3(4+9) = = 3·0 = = 0 (mod 13)

So a(4k+2) can never be prime.

We will inform about progress (if any) about a(4k)

Regards. Jose Brox