Goldbach's conjecture and number of ways in which an even number can be expressed as a sum of two primes Is there a functon that counts the number of ways in which an even number can be expressed as a sum of two primes?

Yes, there is such a function and it has been studied for at least a century. See Sloane's A002375. But it doesn't have a set letter specified for it. Here I will use $g$. So, for example, $g(36) = 4$, $g(38) = 2$. (If you're looking in Sloane's, be sure to divide by 2 before looking up). There is also a function which requires the primes to be distinct, so $31 + 7 = 38$ counts but $19 + 19$ does not; that has also been studied for at least a century.
Now, is there a formula that you can plug in an even number $2n$ and have it give you an answer without knowing the primes up to $n$? I think that if the Riemann hypothesis is proven, it could lead to such a formula. As a quick and dirty estimate, I suggest $g(2n) \approx \frac{n}{8}$; your mileage may vary.
EDIT: Gerry Myerson rightly pointed out that $g(2n) < \pi(n)$, and that my quick and dirty estimate is quite inadequate for large numbers. From his comment, I revise my quick and dirty estimate to $g(2n) \approx \frac{n}{4 \log n}$. The point that I was getting at is that Bertrand's postulate tells us there is always at least one prime between $n$ and $2n$, and it seems unlikely to me that each and every prime in that interval would fail to "match" to a prime between 1 and $n$.
