An univariate dataset (x, y) of class data.frame and dim 51 x 2 to be fitted by a
neural network with 3 hidden neurons (10 parameters).
This dataset was used to teach neural networks at ESPCI from 1991 to 2013. It usually
appeared in the very first slides.
This is a combination of 3 pure tanh() functions without noise. The Jacobian matrix
is singular at the target parameter values and many algorithms could fail.