A256594 Numbers k such that k!*2^k + 1 is prime.

0, 1, 259, 16708, 18655, 26304, 61999, 110251

Offset

1,3

Links

Table of n, a(n) for n=1..8.

Formula

a(n) = A080778(n+1)/2 for n >= 2. - Amiram Eldar, Dec 06 2018

Example

0 is in the sequence since 0!*2^0 + 1 = 2 is prime.

Mathematica

Select[Range[0, 20000], PrimeQ[2^#*#! + 1] &]

Prog

(Magma) [n: n in [0..3*10^2] | IsPrime(Factorial(n)*2^n+1)]; // Vincenzo Librandi, Apr 05 2015
(PARI) for(n=0, 300, if(ispseudoprime(n!*2^n+1), print1(n, ", "))) \\ Derek Orr, Apr 05 2015
(Python)
from sympy import factorial, isprime
for n in range(0, 300):
    if isprime(factorial(n)*(2**n)+1):
        print(n, end=', ') # Stefano Spezia, Dec 06 2018

Crossrefs

Cf. A007749, A080778, A117141, A091415.

Sequence in context: A344193 A254642 A322882 * A229433 A022221 A121918

Adjacent sequences: A256591 A256592 A256593 * A256595 A256596 A256597

Keyword

nonn,hard,more

Author

Robert Price, Apr 03 2015

Extensions

a(6)-a(8), from the data at A080778, added by Amiram Eldar, Dec 06 2018

Status

approved