Table 2.

The forms of the kernel function.

Kernel Function	Forms
The polynomial kernel function	$\mathrm{K}\left(\mathrm{x},x_i\right)={\left(x·x_i+c\right)}^N$
The radial basis kernel function	$\mathrm{K}\left(\mathrm{x},x_i\right)=\exp \left(\frac{\Vert x-x_i\Vert {}^2}{2{\sigma }^2}\right)$
The sigmoid kernel function	$\mathrm{K}\left(\mathrm{x},x_i\right)=\tanh \left(\tau ·\left(\mathrm{x},x_i\right)+\mathrm{t}\right),(\tau >0,\mathrm{t}>0)$
The linear kernel function	$\mathrm{k}\left(\mathrm{x},x_i\right)=\mathrm{x}\ast x_i$