A231755 Primes of the form (2^n-1)/3 - n.

331, 1398079, 89478457, 393530540239137101071, 1730765619511609209510165443073253, 8173309551284740577911184144801648979299941984979211421, 2142584059011987034055949456454883470029603991710390447068299

Offset

1,1

Comments

a(14) has 671 digits. a(15) has 2820 digits (not included in b-file).

Alternately, primes of the form Jacobsthal(n) - n, where Jacobsthal(n) is the n-th Jacobsthal number.

Links

K. D. Bajpai, Table of n, a(n) for n = 1..14

Example

a(2)= 1398079: n=22: ((2^n-(-1)^n)/3-n)= 1398079, which is prime.
a(4)= 393530540239137101071: n=70: ((2^n-(-1)^n)/3-n)= 393530540239137101071, which is prime.

Maple

KD := proc() local a; a:= (2^n -(-1)^n)/3-n; if isprime(a)then RETURN (a); fi; end: seq(KD(), n=1..1000);

Prog

(PARI) for(n=8, 500, if(ispseudoprime(t=2^n\/3-n), print1(t", "))) \\ Charles R Greathouse IV, Nov 13 2013

Crossrefs

Cf. A001045 (Jacobsthal numbers).

Cf. A107036 (indices of prime Jacobsthal numbers).

Cf. A128209 (Jacobsthal numbers+1).

Sequence in context: A199820 A255389 A133141 * A097401 A376734 A250758

Adjacent sequences: A231752 A231753 A231754 * A231756 A231757 A231758

Keyword

nonn,less

Author

K. D. Bajpai, Nov 13 2013

Extensions

Definition corrected by Charles R Greathouse IV, Nov 13 2013

Status

approved