OFFSET
1,4
COMMENTS
For n > 3, a(n) = 0 if n-2 is an odd composite.
The sequence without zeros is a subsequence of A189553. - Manfred Scheucher, Aug 08 2015
The two prime factors are not necessarily distinct; a(6) = 9, both of whose prime factors are 3s. - Jon E. Schoenfield, Aug 09 2015
FORMULA
EXAMPLE
a(10) = 21 because 21 = 3*7 and 3+7 = 10, and there is no semiprime smaller than 21 whose two prime factors sum to 10.
MAPLE
with(numtheory):for n from 1 to 65 do:ii:=0:for k from 1 to 1000 while(ii=0)do:m1:=bigomega(k):x:=factorset(k): m2:=nops(x):if m1=2 and m2=2 and x[1]+x[2]= n or m1=2 and m2=1 and 2*x[1]= n then ii:=1: printf(`%d, `, k):else fi:od:if ii=0 then printf(`%d, `, 0):else fi:od:
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Nov 20 2011
EXTENSIONS
Edited by Jon E. Schoenfield and Manfred Scheucher, Aug 09 2015
STATUS
approved