Table 1.

Commonly used kernel functions.

Kernel functions	Formula
Linear kernel	κ(xi, xj) = xiTxj
Polynomial kernel	κ(xi, xj) = (xiTxj)d
Gauss kernel	$\kappa \left(x_i,x_j\right)=\exp \left(-\frac{{‖x_i-x_j‖}^2}{2{\sigma }^2}\right)$
Sigmoid kernel	$\kappa \left(x_i,x_j\right)=\exp \left(-\frac{‖x_i-x_j‖}{\sigma }\right)$
Laplace kernel	κ(xi, xj) = tanh(βxiTxj + θ)