OFFSET
1,1
COMMENTS
The numbers that follow the expression in the definition have this form: (12) concatenated n times and prepended to 34567.
Empirical observations: primes alternate with nonprimes. 6th (nonprime) and 7th (prime) terms correspond to probable primes. Up to which term the pattern will hold?
(2^(2*a(n)+7) * 5^(2*a(n)+5) + 740711) has 7 proper divisors.
EXAMPLE
11 appears because 121212121212121212121234567 ('12' concatenated 11 times and prepended to '34567') is prime.
MAPLE
A260903:=n->`if`(isprime((2^(2*n+7) * 5^(2*n+5) + 740711)/33), n, NULL): seq(A260903(n), n=1..500); # Wesley Ivan Hurt, Nov 27 2015
MATHEMATICA
Select[Range[500], PrimeQ[(2^(2# + 7) * 5^(2# + 5) + 740711)/33] &] (* or *)
Select[Range[50], DivisorSigma[0, (2^(2# + 7) * 5^(2# + 5) + 740711)] - 1 == 7 &] (* inefficient *)
PROG
(Magma) [n: n in [1..250] | IsPrime((2^(2*n+7) * 5^(2*n+5) + 740711) div 33)]; // Vincenzo Librandi, Nov 18 2015
(PARI) is(n)=isprime((2^(2*n+7)*5^(2*n+5) + 740711)/33) \\ Anders Hellström, Nov 18 2015
CROSSREFS
KEYWORD
nonn,base,hard,more
AUTHOR
Mikk Heidemaa, Nov 17 2015
STATUS
approved