y=arctanx-ln((1+x^2)^1/2)
y'=1/(1+x^2)-x/(1+x^2)
=1-x^2+x^4-x^6+...
-x+x^3-x^5+.
y=x-x^2/2-x^3/3+x^4/4+x^5/5+.