cp's OEIS Frontend

A096139 Number of ways to write 2*n as an ordered sum of two numbers which are prime or 1.

%I A096139 #12 Dec 13 2022 03:46:58
%S A096139 1,3,3,4,3,4,5,4,6,6,5,8,5,4,8,6,7,8,5,6,10,8,7,12,8,6,12,6,7,14,7,10,
%T A096139 12,6,10,14,11,10,14,10,9,18,9,8,20,8,9,14,8,12,18,12,11,18,14,14,22,
%U A096139 12,11,24,7,10,20,8,14,20,11,10,18,16,15,22,11,10,26,10,16,22,11,16,20,12
%N A096139 Number of ways to write 2*n as an ordered sum of two numbers which are prime or 1.
%C A096139 A001031(n) = floor((a(n)+1)/2); a(n) mod 2 = A010051(n). - _Reinhard Zumkeller_, Aug 28 2013
%H A096139 Reinhard Zumkeller, <a href="/A096139/b096139.txt">Table of n, a(n) for n = 1..10000</a>
%H A096139 <a href="/index/Go#Goldbach">Index entries for sequences related to Goldbach conjecture</a>
%e A096139 a(2)=3 because 4=1+3 or 4=2+2 or 4=3+1;
%e A096139 a(3)=3 because 6=1+5 or 6=3+3 or 6=5+1;
%e A096139 a(4)=4 because 8=1+7 or 8=3+5 or 8=5+3 or 8=7+1;
%e A096139 a(5)=3 because 10=3+7 or 10=5+5 or 10=7+3.
%o A096139 (Haskell)
%o A096139 a096139 n = sum (map a010051 gs') + fromEnum (1 `elem` gs')
%o A096139    where gs' = map (2 * n -) $ takeWhile (< 2 * n) a008578_list
%o A096139 -- _Reinhard Zumkeller_, Aug 28 2013
%o A096139 (PARI) { a(n) = local(s(n) = if(n==1,1,isprime(n))); sum(i=1,2*n,s(i)*s(2*n-i)); } \\ _Christian Krause_, Dec 06 2022
%Y A096139 Cf. A008578, A010051, A001031, A347744.
%K A096139 easy,nonn
%O A096139 1,2
%A A096139 _David Stroup_, Jul 23 2004
%E A096139 Example extended and typo fixed by _Reinhard Zumkeller_, Aug 28 2013