A217837 - OEIS

%I #11 Oct 12 2012 13:06:39

%S 14,23,30,36,40,46,51,54,55,59,62,72,75,76,80,82,85,98,103,104,113,

%T 118,129,132,134,136,138,145,167,170,172,174,195,198,204,225,240,264,

%U 270,275,280,284,285,287,292,301,314,318,329,336,337,339,346,347,348

%N Numbers n such that 4n - 1 and 4n + 1 are both semiprimes.

%C This is to semiprimes as A045753 is to primes.

%H Zak Seidov, <a href="/A217837/b217837.txt">Table of n, a(n) for n = 1..1000</a>

%e a(1) = 14 because 56 - 1 = 5*11 and 56 + 1 = 3*19 are semiprimes, and this is the first number having this property.

%e a(2) = 23 because 92 - 1 = 7*13 and 92 + 1 = 3*31 are semiprimes, and this is the second number having this property.

%e a(3) = 30 because 120 - 1 = 7*17 and 120 + 1 = 11*11 are semiprimes, and this is the third number having this property.

%t Select[Range[500], PrimeOmega[4# - 1] == 2 && PrimeOmega[4# + 1] == 2 &] (* _Alonso del Arte_, Oct 12 2012 *)

%Y Cf. A045753.

%K nonn

%O 1,1

%A _Zak Seidov_, Oct 12 2012