OFFSET
1,1
COMMENTS
Except for case p=5, p+4 is never a perfect square.
For p = {2, 11, 31, 73, 139, 433, 1759, 2017} p+4 is a product of two consecutive primes.
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
EXAMPLE
2+4=6=2*3, 5+4=9=3*3, 11+4=15=3*5 (all semiprimes).
MATHEMATICA
Select[Prime@ Range@ 150, PrimeOmega[# + 4] == 2 &] (* Michael De Vlieger, Jun 29 2017 *)
PROG
(PARI) issemi(n)=bigomega(n)==2
is(n)=isprime(n) && issemi(n+4) \\ Charles R Greathouse IV, Jul 02 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Jun 29 2017
STATUS
approved