A138595 Numbers k such that 2^(2*k-21) - 21 is prime.

13, 14, 15, 16, 17, 21, 22, 31, 55, 205, 217, 255, 445, 805, 867, 1426, 5432, 5639, 8485, 12267, 19821, 21685, 31481, 69629

Offset

1,1

Links

Table of n, a(n) for n=1..24.

Example

2^(2*13-21)-21=11, 2^(2*14-21)-21=107, 2^(2*15-21)-21=491, 2^(2*16-21)-21=2027, ...

Mathematica

q=21; For[i=q, i<=q, a={}; Do[x=2^(2*n-i)-i; If[PrimeQ[x], AppendTo[a, n]], {n, 10^2}]; Print["2^(2*n-", i, ")-", i, " ", a]; i=i+2]

Prog

(PARI) is(n)=isprime(2^(2*n-21)-21) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

Cf. A138577, A138578, A138579, A138580, A138581, A138582.

Cf. A138596, A138598, A138599, A138600, A138601, A138602, A138603.

Sequence in context: A178402 A132580 A293831 * A118140 A108854 A106007

Adjacent sequences: A138592 A138593 A138594 * A138596 A138597 A138598

Keyword

nonn,more

Author

Vladimir Joseph Stephan Orlovsky, May 13 2008

Extensions

11 and 12 removed by Sean A. Irvine, May 21 2026

a(13)-a(24) from Michael S. Branicky, May 22 2026

Status

approved