Table 2.

The four basic kernel functions

Kernel name	Mathematical formulation
Linear	$K({\mathit{x}}_i,{\mathit{x}}_j)={\mathit{x}}_i^{⊺}{\mathit{x}}_j$
Polynomial	$K({\mathit{x}}_i,{\mathit{x}}_j)={\left(\gamma {\mathit{x}}_i^{⊺}{\mathit{x}}_j+r\right)}^d,\gamma >0$
Radial basis function (RBF)	K(xi,xj)=exp(−γ∥xi−xj∥2),γ>0
Sigmoid	$K({\mathit{x}}_i,{\mathit{x}}_j)=\text{tanh}\left(\gamma {\mathit{x}}_i^{⊺}{\mathit{x}}_j+r\right),\gamma >0$

γ, r and d are kernel parameters