A217348 Numbers k such that 4^k - 5 is prime.

2, 3, 4, 5, 6, 9, 10, 13, 16, 18, 28, 33, 59, 65, 75, 83, 103, 113, 275, 353, 405, 568, 614, 909, 1184, 1200, 1564, 2266, 2556, 4246, 8014, 8193, 8696, 9291, 10993, 12146, 13809, 15459, 16381, 24106, 60220, 91816, 158070, 182491, 207016, 266675, 297561

Offset

1,1

Links

Table of n, a(n) for n=1..47.

Formula

a(n) = A059608(n+1)/2. - Daniel Starodubtsev, Mar 20 2020

Example

28 is a term because 4^28 - 5 = 72057594037927931 is prime.

Mathematica

Select[Range[10000], PrimeQ[4^# - 5] &]

Prog

(PARI) /* Up to 620 the code produces in few seconds the first terms: */
allocatemem(10000000); for(n=2, 620, if(isprime(4^n-5), print1(n", ")));

Crossrefs

Cf. A059266, A059608, A059614.

Sequence in context: A018321 A178971 A082642 * A319450 A153013 A052492

Adjacent sequences: A217345 A217346 A217347 * A217349 A217350 A217351

Keyword

nonn,more

Author

Vincenzo Librandi, Oct 01 2012

Extensions

a(31)-a(34) from Bruno Berselli, Oct 02 2012

a(35)-a(45) from Daniel Starodubtsev, Mar 20 2020

a(46)-a(47) derived from A059608 by Elmo R. Oliveira, Nov 28 2023

Status

approved