A341713 Indices of Ennesrem primes: k such that A004094(k)-1 is prime.

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

Offset: 1

Author

N. J. A. Sloane, Feb 20 2021

Keywords

• nonn
• base
• more

Comments

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

Is this sequence infinite?

Examples

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

Links

• N. J. A. Sloane, Exciting Number Sequences (video of talk), Mar 05 2021.

Crossrefs

Cf. A000043, A004094, A057708, A341709.

Programs

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

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