Sigmoid (hyperbolic tangent) kernel.
See http://crsouza.com/2010/03/kernel-functions-for-machine-learning-applications/#sigmoid

npm i ml-kernel-sigmoid

Options:

• alpha - value for the alpha factor (default: 0.01).
• constant - value for the constant (default: -Math.E).

Returns the dot product between x and y in feature space.

MIT