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 A072931 Number of ways to write n as a sum of 2 semiprimes. 7

%I

%S 0,0,0,0,0,0,0,1,0,1,0,1,1,1,1,1,0,2,2,2,1,0,1,2,2,1,1,2,2,3,3,2,0,1,

%T 3,3,2,1,3,3,2,3,4,4,2,1,4,5,3,3,1,3,3,2,5,3,2,2,5,6,6,1,3,5,3,4,4,5,

%U 3,3,6,7,5,3,3,4,4,4,5,5,3,2,7,7,2,4,4,5,4,6,8,6,3,3,8,7,7,4,6,8,6,5,7,7,2

%N Number of ways to write n as a sum of 2 semiprimes.

%C Sequence is probably >0 for n> 33.

%C The graph of this sequence is compelling evidence that 33 is the last term of sequence A072966. - T. D. Noe, Apr 10 2007

%C A072931(A100592(n))=n and A072931(m)<n for m < A100592(n); A171963(n)=a(A001358(n)). [From _Reinhard Zumkeller_, Jan 21 2010]

%H T. D. Noe, <a href="/A072931/b072931.txt">Table of n, a(n) for n=1..10000</a>

%o (PARI) a(n)=sum(i=1,n,sum(j=1,i,if(abs(bigomega(i)-2)+abs(bigomega(j)-2)+abs(n-i-j),0,1)))

%o (PARI) a(n)=my(s); forprime(p=2,n\4, forprime(q=2,min(n\(2*p),p), if(bigomega(n-p*q)==2, s++))); s \\ _Charles R Greathouse IV_, Dec 07 2014

%Y Cf. A001358, A072966, A101048.

%K easy,nonn,changed

%O 1,18

%A _Benoit Cloitre_, Aug 13 2002

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