OFFSET
1,1
COMMENTS
Sum_{n>=2} 1/a(n) > 0.02187310784. - R. J. Mathar, Jan 23 2013
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Cousin Primes
EXAMPLE
a(1) = 3*7 = 3*(3+4) = 21;
a(2) = 7*11 = 7*(7+4) = 77;
a(3) = 13*17 = 13*(13+4) = 221;
a(4) = 19*23 = 19*(19+4) = 437.
MATHEMATICA
fQ[n_] := Block[{fi = FactorInteger@ n}, Last@# & /@ fi == {1, 1} && Differences[ First@# & /@ fi] == {4}]; Select[ Range@ 700000, fQ] (* Robert G. Wilson v, Feb 08 2012 *)
PROG
(Haskell)
a143206 n = a143206_list !! (n-1)
a143206_list = (3*7) : f a000040_list where
f (p:ps@(p':_)) | p'-p == 4 = (p*p') : f ps
| otherwise = f ps
-- Reinhard Zumkeller, Sep 13 2011
(Magma) [(p*(p+4)): p in PrimesUpTo(1000)| IsPrime(p+4)]; // Vincenzo Librandi, Jan 04 2018
(PARI) lista(nn) = forprime(p=2, nn, if (isprime(q=p+4), print1(p*q, ", "))); \\ Michel Marcus, Jan 04 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, Aug 12 2008
STATUS
approved