Mercer Kernel Theorem
[Symmetric positive semi-definite](https://texonom.com/symmetric-positive-semi-definite-16ce5c2766e94f3d803ea6ddf2742660) 


1. Symmetric: K(x, z) = K(z, x)
2. Positive semi-definite: for any finite set of points {x1, x2, ..., xn}, the corresponding kernel matrix K must be symmetric and positive semi-definite









