%N Numbers n such that (10^n - 3^n)/7 is prime.
%C All terms are primes.
%C No other terms < 100000.
%t k=7; Select[ Prime[ Range[1,200] ], PrimeQ[ ((k+3)^# - 3^#)/k ]& ]
%o (PARI) forprime(p=2,1e4,if(ispseudoprime((10^p-3^p)/7),print1(p", "))) \\ _Charles R Greathouse IV_, Jun 05 2011
%Y Cf. A028491 = numbers n such that (3^n - 1)/2 is prime.
%Y Cf. A057468 = numbers n such that 3^n - 2^n is prime.
%Y Cf. A059801 = numbers n such that 4^n - 3^n is prime.
%Y Cf. A121877 = numbers n such that (5^n - 3^n)/2 is a prime.
%Y Cf. A128024, A128025, A128027, A128028, A128029, A128030, A128031, A128032.
%A _Alexander Adamchuk_, Feb 11 2007
%E a(6)-a(7) from _Robert Price_, Jun 04 2011