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A341713
Indices of Ennesrem primes: k such that A004094(k)-1 is prime.
1
2, 3, 13, 21, 347, 1217, 1267, 16459, 100909, 342243
OFFSET
1,1
COMMENTS
Numbers k such that reverse(2^k) - 1 is prime.
Is this sequence infinite?
LINKS
N. J. A. Sloane, Exciting Number Sequences (video of talk), Mar 05 2021.
EXAMPLE
13 is a term, since 2^13 = 8192 -> 2918 -> 2917, which is prime.
PROG
(PARI) for(n=1, 10000, my(pe=fromdigits(Vecrev(digits(2^n)))-1); if(ispseudoprime(pe), print1(n, ", "))) \\ Hugo Pfoertner, Feb 20 2021
(Python)
from sympy import isprime
def ok(k): return isprime(int(str(2**k)[::-1]) - 1)
for k in range(1, 2*10**3):
if ok(k): print(k, end=", ") # Michael S. Branicky, Feb 20 2021
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
N. J. A. Sloane, Feb 20 2021
EXTENSIONS
a(8) from Hugo Pfoertner, Feb 20 2021
a(9) from Hugo Pfoertner, Mar 22 2021
a(10) from Steven Charlton, Apr 26 2021
STATUS
approved