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A180041 Number of Goldbach partitions of (2n)^n. 1

%I

%S 0,2,13,53,810,20564,274904,6341424,419586990

%N Number of Goldbach partitions of (2n)^n.

%C This is the main diagonal of the array mentioned in A180007, only considering even rows (as odd numbers cannot be the sums of two odd primes), namely A(2n, n) = number of ways of writing (2n)^n as the sum of two odd primes, when the order does not matter.

%F a(n) = A061358((2*n)^n) = A061358(A062971(n)).

%e a(1) = 0 because 2*1 = 2 is too small to be the sum of two primes.

%e a(2) = 2 because 4^2 = 16 = 3+13 = 5+11.

%e a(3) = 13 because 6^3 = 216 and A180007(3) = Number of Goldbach partitions of 6^3 = 13.

%e a(4) = 53 because 8^4 = 2^12 and A006307(12) = Number of ways writing 2^12 as unordered sums of 2 primes.

%p A180041 := proc(n) local a,m,p: if(n=1)then return 0:fi: a:=0: m:=(2*n)^n: p:=prevprime(ceil((m-1)/2)): while p > 2 do if isprime(m-p) then a:=a+1: fi: p := prevprime(p): od: return a: end: seq(A180041(n),n=1..5); # _Nathaniel Johnston_, May 08 2011

%t f[n_] := Block[{c = 0, p = 3, m = (2 n)^n}, lmt = Floor[m/2] + 1; While[p < lmt, If[ PrimeQ[m - p], c++ ]; p = NextPrime@p]; c]; Do[ Print[{n, f@n // Timing}], {n, 8}] (* _Robert G. Wilson v_, Aug 10 2010 *)

%Y Cf. A001031, A061358, A065577, A180007.

%K more,nonn

%O 1,2

%A _Jonathan Vos Post_, Aug 07 2010

%E a(6)-a(8) from _Robert G. Wilson v_, Aug 10 2010

%E a(9) from _Giovanni Resta_, Apr 15 2019

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Last modified September 24 23:13 EDT 2020. Contains 337325 sequences. (Running on oeis4.)