A177997 p and A002808(p)/2 are both prime.

2, 5, 7, 13, 31, 41, 43, 59, 101, 107, 127, 137, 149, 199, 239, 277, 359, 389, 479, 613, 743, 757, 809, 829, 937, 991, 1031, 1103, 1439, 1487, 1499, 1847, 1877, 2011, 2083, 2179, 2609, 2663, 2711, 2741, 2749, 2857, 2909, 3329, 3559, 3623, 3643, 3697, 3823

Offset

1,1

Comments

Primes p such that composite(p) is an even semiprime.

Links

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

Example

a(1)=2 because 2=prime and composite(2)/2=6/2=3=prime.

Maple

P, C:= selectremove(isprime, [$2..10000]):
select(t -> t <= nops(C) and C[t]::even and isprime(C[t]/2), P); # Robert Israel, Mar 20 2018

Prog

(PARI)
c=[]; for(n=2, 10000, if(!isprime(n), c=concat(c, n))); c; \\ The composites
s=[]; forprime(p=2, #c, if(c[p]%2==0 && isprime(c[p]\2), s=concat(s, p))); s \\ Colin Barker, Jun 28 2014

Crossrefs

Cf. A002808, A065858, A065897.

Sequence in context: A250173 A038945 A215210 * A238776 A141112 A053647

Adjacent sequences: A177994 A177995 A177996 * A177998 A177999 A178000

Keyword

nonn

Author

Juri-Stepan Gerasimov, May 17 2010, May 23 2010

Extensions

Corrected by D. S. McNeil and R. J. Mathar, May 23 2010

Status

approved