Talk:Differentiation of Power Series

As already discussed on p.ms talk page, and brought up by abcxyz, we need to justify termwise differentiation. One possible track is to start introducing formal power series, and then showing that such a power series converges on the open disc with the convergence radius, and then that its formal (termwise) derivative also has the same radius of convergence. --Lord_Farin 02:55, 8 March 2012 (EST)