

A104279


Numbers n such that 2n+1 is prime and 2n1 is not prime.


3



1, 5, 8, 11, 14, 18, 20, 23, 26, 29, 33, 35, 39, 41, 44, 48, 50, 53, 56, 63, 65, 68, 74, 78, 81, 83, 86, 89, 95, 98, 105, 111, 113, 116, 119, 125, 128, 131, 134, 138, 140, 146, 153, 155, 158, 165, 168, 173, 176, 179, 183, 186, 189, 191, 194, 198, 200, 204, 209, 215
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OFFSET

1,2


LINKS

Robert Israel, Table of n, a(n) for n = 1..10000


EXAMPLE

a(1)=1 because 3 is prime and 1 is not prime.
a(2)=5 because 11 is prime and 9 is not prime.


MAPLE

filter:= n > isprime(2*n+1) and not isprime(2*n1):
select(filter, [$1..300]); # Robert Israel, Apr 27 2020


MATHEMATICA

Select[ Range[218], PrimeQ[2# + 1] && !PrimeQ[2#  1] &] (* Robert G. Wilson v, Apr 18 2005 *)


PROG

(MAGMA) [n: n in [1..220] not IsPrime(2*n1) and IsPrime(2*n+1)] // Vincenzo Librandi, Jan 28 2011
(PARI) is(n)=isprime(2*n+1) && !isprime(2*n1) \\ Charles R Greathouse IV, Jun 13 2017


CROSSREFS

Cf. A040040 and others.
Sequence in context: A314388 A022798 A022808 * A184919 A063247 A184928
Adjacent sequences: A104276 A104277 A104278 * A104280 A104281 A104282


KEYWORD

easy,nonn


AUTHOR

Alexandre Wajnberg, Apr 17 2005


EXTENSIONS

More terms from Robert G. Wilson v, Apr 18 2005


STATUS

approved



