A104278 - OEIS

A104278

Numbers m such that 2m+1 and 2m-1 are not primes.

13, 17, 25, 28, 32, 38, 43, 46, 47, 58, 59, 60, 61, 62, 67, 71, 72, 73, 77, 80, 85, 88, 92, 93, 94, 101, 102, 103, 104, 107, 108, 109, 110, 118, 122, 123, 124, 127, 130, 133, 137, 143, 144, 145, 148, 149, 150, 151, 152, 160, 161, 162, 163, 164, 167, 170, 171, 172

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

Complement of A147820. - Omar E. Pol, Nov 17 2008

m is in the sequence iff A177961(m)<m. - Vladimir Shevelev, May 16 2010

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

EXAMPLE

a(1)=13 is the first number satisfying simultaneously the two rules.

MATHEMATICA

Select[ Range[300], !PrimeQ[2# + 1] && !PrimeQ[2# - 1] &] (* Robert G. Wilson v, Apr 18 2005 *)

Select[Range[300], NoneTrue[2#+{1, -1}, PrimeQ]&] (* The program uses the NoneTrue function from Mathematica version 10 *) (* Harvey P. Dale, Jul 07 2015 *)

PROG

(Haskell)

a104278 n = a104278_list !! (n-1)

a104278_list = [m | m <- [1..],

a010051' (2 * m - 1) == 0 && a010051' (2 * m + 1) == 0]

-- Reinhard Zumkeller, Aug 04 2015

(PARI) select( {is_A104278(n)=!isprime(2*n-1)&&!isprime(2*n+1)}, [1..222]) \\ M. F. Hasler, Apr 29 2024

CROSSREFS

Intersection of A047845 and A104275.

Cf. A040040, A147820.

Cf. A010051, A002808, A099047.

Sequence in context: A349978 A125262 A163754 * A129070 A335036 A307880