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A181490

Numbers k such that 3*2^k-1 and 3*2^k+1 are twin primes (A001097).

1, 2, 6, 18

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OFFSET

1,2

COMMENTS

Sequences A181491 and A181492 list the corresponding primes.

No more terms below three million. - Charles R Greathouse IV, Mar 14 2011

Intersection of A002235 and A002253. - Jeppe Stig Nielsen, Mar 05 2018

LINKS

Table of n, a(n) for n=1..4.

FORMULA

Equals { k | A007283(k) in A014574 } = { k | A153893(k) in A001359 }.

MAPLE

a:=k->`if`(isprime(3*2^k-1) and isprime(3*2^k+1), k, NULL); seq(a(k), k=1..1000); # Muniru A Asiru, Mar 11 2018

MATHEMATICA

fQ[n_] := PrimeQ[3*2^n - 1] && PrimeQ[3*2^n + 1]; k = 1; lst= {}; While[k < 15001, If[fQ@k, AppendTo[lst, k]; Print@k]; k++ ] (* Robert G. Wilson v, Nov 05 2010 *)

Select[Range[20], AllTrue[3*2^#+{1, -1}, PrimeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Sep 24 2014 *)

PROG

(PARI) for( k=1, 999, ispseudoprime(3<<k-1)||next; ispseudoprime(3<<k+1)&print(k))

(GAP) Filtered([1..300], k->IsPrime(3*2^k-1) and IsPrime(3*2^k+1)); # Muniru A Asiru, Mar 11 2018

CROSSREFS

Cf. A001097, A001359, A002235, A002253, A006512, A014574, A294730.

Sequence in context: A327698 A183757 A356011 * A340210 A165325 A316765

Adjacent sequences: A181487 A181488 A181489 * A181491 A181492 A181493

KEYWORD

bref,hard,more,nonn

AUTHOR

M. F. Hasler, Oct 30 2010

EXTENSIONS

Pari program repaired by Charles R Greathouse IV, Mar 14 2011

STATUS

approved