login

Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.

a(n) = the least semiprime such that n+a(n) is also semiprime.
0

%I #5 Feb 14 2015 23:47:49

%S 9,4,6,6,4,4,14,6,6,4,4,9,9,21,6,6,4,4,6,6,4,4,10,9,9,9,6,6,4,4,4,6,6,

%T 4,4,10,9,39,10,6,10,4,6,14,4,9,4,9,6,15,4,6,4,4,10,6,25,4,6,9,4,15,6,

%U 10,4,21,10,6,22,4,6,10,4,21,10,6,9,4,6,6,4,4,4,9,6,9,4,6,4,4,4,14

%N a(n) = the least semiprime such that n+a(n) is also semiprime.

%C Corresponding values of n+a(n): 10,6,9,10,9,10,21,14,15,14,15,21,22,35,21.

%e a(1)=9 because 1+9=10=2*5, a(2)=4 because 2+4=6=2*3.

%t Table[m=4;While[2!=PrimeOmega[m]||2!=PrimeOmega[m+n],m++];m,{n,100}]

%Y Cf. A001358.

%K nonn

%O 1,1

%A _Zak Seidov_, Feb 12 2015