A002238 Numbers k such that 21*2^k - 1 is prime.

1, 2, 3, 7, 10, 13, 18, 27, 37, 51, 74, 157, 271, 458, 530, 891, 1723, 1793, 1849, 1986, 2191, 2869, 4993, 7777, 11730, 15313, 29171, 35899, 36227, 71570, 199219, 233914, 297499, 332523, 348547, 538657, 986130, 999599

Offset: 1

Author

N. J. A. Sloane

Keywords

• hard
• nonn

References

• H. Riesel, Lucasian criteria for the primality of N=h.2^n-1, Math. Comp., 23 (1969), 869-875.
• N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
• N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

• Ray Ballinger and Wilfrid Keller, List of primes k.2^n + 1 for k < 300
• R. K. Guy, The strong law of small numbers. Amer. Math. Monthly 95 (1988), no. 8, 697-712. [Annotated scanned copy]
• Wilfrid Keller, List of primes k.2^n - 1 for k < 300
• Kosmaj, Riesel list k<300.
• Index entries for sequences of n such that k*2^n-1 (or k*2^n+1) is prime

Crossrefs

Cf. A032360, 21*2^k + 1 is prime.

Programs

PARI

is(n)=ispseudoprime(21*2^n-1) \\ Charles R Greathouse IV, May 22 2017

Extensions

More terms from Hugo Pfoertner, Jun 22 2004

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008