Mathematica で高次の項を無視する

https://stackoverflow.com/questions/21517020/in-mathematica-how-can-i-cut-off-the-high-order-terms-in-a-polynomial

In[1]:= y = a0 + a1*x + a2*x^2 + a3*x^3 + a4*x^4;
y /. x^b_ /; b >= 3 -> 0

Out[2]= a0 + a1 x + a2 x^2