equivalent command in sage?

edit

By the way, here is an interact I made to find out the Fourier series of a piecewise defined function: http://flask.sagenb.org/home/pub/90/

Thanks you for the answer, that helps a lot. I was trying to create an interactive applet using sagenb, but still getting used to Sage from mathematica.

I know how you feel: I also was a Mathematica user before getting into Sage. It is worth the effort :)