OFFSET
1,3
COMMENTS
Either (but not both) of 3^n - 2^n and 3^n + 2^n is prime. - Harvey P. Dale, Sep 16 2016
If 3^n + 2^n is prime then n must be a power of 2, and 3^n + 2^n is a generalized Fermat prime. It is conjectured that 3^n + 2^n is prime only for n=1,2,4: see A082101. - Robert Israel, Mar 15 2017, edited May 18 2017.
EXAMPLE
a(1)=0 because 3^0 - 2^0 = 0 = nonprime and 3^0 + 2^0 = 2 = prime;
a(2)=1 because 3^1 - 2^1 = 1 = nonprime and 3^1 + 2^1 = 5 = prime;
a(3)=3 because 3^3 - 2^3 = 19 = prime and 3^3 + 2^3 = 35 = nonprime.
MATHEMATICA
epQ[n_]:=Module[{a=3^n, b=2^n}, Sort[PrimeQ[{a+b, a-b}]]=={False, True}]; Select[Range[0, 4000], epQ] (* Harvey P. Dale, Sep 16 2016 *)
PROG
(PARI) is(n)=isprime(3^n+2^n)+isprime(3^n-2^n)==1 \\ Charles R Greathouse IV, Mar 19 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Juri-Stepan Gerasimov, Mar 15 2010
EXTENSIONS
9 and 11 removed by R. J. Mathar, Mar 29 2010
More terms from Harvey P. Dale, Sep 16 2016
a(20) from Robert G. Wilson v, Mar 15 2017
a(21) to a(29) (using data from A057468) from Robert Israel, May 18 2017
STATUS
approved