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%I #33 Sep 08 2022 08:46:14
%S 1,2,4,7,14,17,55,61,259,269,791,3613,6448,8317,18194,32219
%N Numbers n such that (10^(n+4)*7 - 36763)/9 is prime (n > 0).
%C Numbers n such that '3693' appended to n times the digit 7 is prime. Up to a(15) nonprimes alternate with primes.
%C A(n) mod 3 -> {1, 2, 1, 1, 2, 2, 1, 1, 1, 2, 2, 1, 1, 1, 2, 2, ...?}.
%e 4 appears because 77773693 is prime ('7' concatenated 4 times and prepended to '3693') is prime).
%p A265309:= n->`if`(isprime((10^(n + 4) * 7 - 36763)/9), n, NULL):
%p seq(A265309(n), n=1..1000);
%t Select[ Range[10^3], PrimeQ[(10^(# + 4) * 7 - 36763)/9] &]
%o (Magma)[n: n in[1 .. 1000] | IsPrime((10^(n+4) * 7 - 36763) div 9)];
%o (PARI) is(n)=isprime((10^(n+4)*7 - 36763)/9)
%Y Cf. A260903.
%K nonn,base,more
%O 1,2
%A _Mikk Heidemaa_, Dec 06 2015
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