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A177997 p and A002808(p)/2 are both prime. 1
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified March 23 05:36 EDT 2019. Contains 321422 sequences. (Running on oeis4.)