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%I #42 Nov 05 2017 13:59:29
%S 5,7,9,8,10,13,15,14,19,12,21,16,25,20,16,22,31,33,18,26,39,18,43,22,
%T 45,32,20,34,49,24,55,40,28,61,24,63,44,46,26,69,50,73,24,34,75,36,81,
%U 56,30,85,62,91,64,42,28,99,70,103,36,46,105,30,74,109,48,38,111
%N Sum of the distinct prime factors of the squarefree semiprimes (A006881).
%C Sum of the distinct prime factors of A006881(n). If A006881(n) is even then a(n) = A006881(n)/2 + 2. If A006881(n) is odd then a(n) is even.
%H Michael De Vlieger, <a href="/A228578/b228578.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = sopf(A006881(n)) = A008472(A006881(n)).
%F Also, a(n) = sopfr(A006881(n)) = A001414(A006881(n)) because A006881 are squarefree. - _Zak Seidov_, Oct 28 2015
%e a(1) = 5, since 6 is the first squarefree semiprime and the sum of the distinct prime factors of 6 is 2 + 3 = 5. a(2) = 7 since 10 is the second squarefree semiprime and the sum of the distinct prime factors of 10 is 2 + 5 = 5.
%t Total[First /@ FactorInteger@ #] & /@ Select[Range@ 240, PrimeNu@ # == 2 && SquareFreeQ@ # &] (* _Michael De Vlieger_, Oct 28 2015 *)
%o (PARI) do(x)=my(v=List()); forprime(p=3,x\2, forprime(q=2,min(x\p,p-1), listput(v,[p*q,p+q]))); v=vecsort(Vec(v),1); apply(u->u[2],v) \\ _Charles R Greathouse IV_, Nov 05 2017
%Y Cf. A006881, A001414, A008472.
%K nonn,easy
%O 1,1
%A _Wesley Ivan Hurt_, Aug 28 2013
%E a(61)-a(67) corrected by _Michael De Vlieger_, Oct 28 2015
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