A309607 Exponents k for which reversal(2^k-1) is prime.

2, 3, 5, 53, 189, 293, 1107, 2181, 2695, 2871, 7667, 19999, 27471, 44537, 62323, 134367, 174295

Offset: 1

Author

Metin Sariyar, Aug 09 2019

Keywords

• nonn
• base
• more

Comments

According to the statements in the first link given below, the terms for k <= 7667, the primes of the form reversal(2^k-1) are certified and for k >= 19999 they are probable primes.

Examples

			5 is included because for n=5, reversal(2^5-1)=13 is prime.
		

Links

• Carlos Rivera, Puzzle 417.
• Chris K. Caldwell and G. L. Honaker, Prime Curio for 1990474529917009.

Crossrefs

Cf. A000225, A000668, A001348, A000079, A000043, A057708, A134037, A134038, A134039.

Programs

Mathematica

Select[Range[7667], PrimeQ[IntegerReverse[2^# - 1]] &]

PARI

isok(k) = isprime(fromdigits(Vecrev(digits(2^k-1)))); \\ Michel Marcus, Aug 10 2019