# What is the posterior distribution $p(\textbf{f} | \textbf{y})$ for a Gaussian Process regression?

What is the posterior distribution $$p(\textbf{f} | \textbf{y})$$ for a Gaussian Process regression?

Suppose that $$p(y_n |x_n, f) = N(f(x_n), \sigma^2)$$ with prior on $$\textbf{f} = [f(x_1), \ldots f(x_n)]^T$$ as $$p(\textbf{f}) = N(0, K)$$ where $$K = K(X, X)$$ is the $$n\times n$$ Kernel matrix. How can I derive the expression for the posterior $$f(\textbf{f} | \textbf{y})$$?

It is not clear to me whether the posterior is again going to be a Gaussian Process or it is going to be a Gaussian. Please help.

Note: I am not asking about predictive posterior which every books seem to discuss. None of the books even mention the posterior I am looking for

• Yes the posterior will be Gaussian, and you can use a combination of Bayes theorem and completing the square to show this. – microhaus Apr 10 at 14:29
• @microhaus These notes say otherwise, Slide 4/6 mlg.eng.cam.ac.uk/teaching/4f13/1920/gp%20and%20data.pdf – Abhay Apr 10 at 14:33
• Unless there is something I have missed, I am unable to understand how on slide 4/6, "Gaussian process prior with zero mean and covariance function $k$ and likelihood [in slides] leads to a Gaussian process posterior" amounts to saying otherwise? – microhaus Apr 10 at 14:37
• Also, the predictive posterior that every book seems to discuss and the posterior you are looking for are the same. It is simply the conditional Gaussian distribution where $y_1,...,y_n$ are fixed at $x_1,...,x_n$ and we find posterior distribution (or conditional distribution) at any new $x_i, x_j, ...$. – Zeel B Patel May 6 at 17:02