# Gaussian processes¶

Gaussian processes model a probability distribution over functions.

Let be some function mapping vectors to vectors. Then we can write: where represents the mean vector: and is the kernel function.

## Kernel function¶

The kernel is a function that represents the covariance function for the Gaussian process.  The kernel can be thought of as a prior for the shape of the function, encoding our expectations for the amount of smoothness or non-linearity.

Not all conceivable kernels are valid. The kernel must produce covariance matrices that are positive-definite.

### Linear kernel¶ Some functions sampled from a Gaussian process with a linear kernel: ### Polynomial kernel¶ Functions sampled from a Gaussian process with a polynomial kernel where and : ### Gaussian kernel¶

Also known as the radial basis function or RBF kernel. Some functions sampled from a GP with a Gaussian kernel: ### Laplacian kernel¶ Functions sampled from a GP with a Laplacian kernel: ## Sampling from a Gaussian process¶

The method is as follows:

1. Decide on a vector of inputs for which we want to compute , where is some function which we will sample from the Gaussian process.
2. Compute the matrix where .
3. Perform Cholesky decomposition on , yielding a lower triangular matrix .
4. Sample a vector of numbers from a standard Gaussian distribution, .
5. Take the dot product of and the vector to get the samples .