%I #45 May 29 2023 11:38:08

%S 2,3,13,21,347,1217,1267,16459,100909,342243

%N Indices of Ennesrem primes: k such that A004094(k)-1 is prime.

%C Numbers k such that reverse(2^k) - 1 is prime.

%C Is this sequence infinite?

%H N. J. A. Sloane, <a href="https://www.youtube.com/watch?v=9ogbsh8KuEM">Exciting Number Sequences</a> (video of talk), Mar 05 2021.

%e 13 is a term, since 2^13 = 8192 -> 2918 -> 2917, which is prime.

%o (PARI) for(n=1,10000,my(pe=fromdigits(Vecrev(digits(2^n)))-1);if(ispseudoprime(pe),print1(n,", "))) \\ _Hugo Pfoertner_, Feb 20 2021

%o (Python)

%o from sympy import isprime

%o def ok(k): return isprime(int(str(2**k)[::-1]) - 1)

%o for k in range(1, 2*10**3):

%o if ok(k): print(k, end=", ") # _Michael S. Branicky_, Feb 20 2021

%Y Cf. A000043, A004094, A057708, A341709.

%K nonn,base,more

%O 1,1

%A _N. J. A. Sloane_, Feb 20 2021

%E a(8) from _Hugo Pfoertner_, Feb 20 2021

%E a(9) from _Hugo Pfoertner_, Mar 22 2021

%E a(10) from _Steven Charlton_, Apr 26 2021