# Simplifying expressions involving Sum

I am trying to use Mathematica to simplify a symbolic expression involving Sum. The expression is defined as follows:

y = (x - x0)^α Sum[a[n] (x - x0)^n, {n, 0, Infinity}]


I am trying to use FullySimplify on the derivative of the expression with respect to x via

FullSimplify[D[y, x]]


This yields $$(x-\text{x0})^{\alpha -1} \left(\alpha \sum _{n=0}^{\infty } a(n) (x-\text{x0})^n+(x-\text{x0}) \sum _{n=0}^{\infty } n a(n) (x-\text{x0})^{n-1}\right)$$ However, the expression above can be easily simplified further to $$(x-\text{x0})^{\alpha -1} \sum _{n=0}^{\infty } ( a(n) (x-\text{x0})^n(\alpha+n) )$$

Is there a way to "make" Mathematica recognise this simplification? I presume the problem has something to do with the fact that I use the unknown function a[n] in the expression, but I am not sure what can I do about it to get similar functionality.

I am new to Mathematica and would like to apologise if this is a trivial question.

• Why don't you take the x dependence sitting outside into the sum y = Sum[(x - x0)^[Alpha] *a[n] (x - x0)^n, {n, 0, Infinity}] Commented Aug 26, 2013 at 13:45

In our case a simple and direct approach would be defining a list of rules. Here is an example:

rules =
{ c_ Sum[n a[n] c_^(n-1), {n, 0, Infinity}] :> Sum[n c^n a[n], {n, 0, Infinity}],
α_ Sum[a[n] c_^n, {n, 0, Infinity}] + Sum[n a[n] c_^n, {n, 0, Infinity}] :>
Sum[(α + n) a[n] c^n, {n, 0, Infinity}]};


Let's define an appropriate function for TransformationFunctions applying rules to an expression:

 tf[expr_] := expr /. rules


and now FullSimplify with tf does the expected transformation:

FullSimplify[ D[y, x], TransformationFunctions -> {Automatic, tf}]//TraditionalForm


alternatively one can do this:

FullSimplify[ D[y, x]] //. rules


Note: Applying rules in TransformationFunctions it was quite sufficient to play with ReplaceAll (/.) while in the latter case we had to use rules repeatedly i.e. with ReplaceRepeated (//.) .

One way to do it is application of series of rules to the expression, just to "teach" Mathematica of how to transform the expression. I will break the chain of transformations into steps to make it better visible. These are your functions:

 expr1 = (x - x0)^α  Sum[a[n] (x - x0)^n, {n, 0, Infinity}]
expr2 = D[expr1, x]


and here are the results:

Let us do the first transformation:

expr3 = expr2 /. u_*\!$$\*UnderoverscriptBox[\(\[Sum]$$, $$n = 0$$, $$\[Infinity]$$]v_\) -> \!$$\*UnderoverscriptBox[\(\[Sum]$$, $$n = 0$$, $$\[Infinity]$$]$$(u* v)$$\)


this is the result:

The second transformation:

 expr4 = expr3 /. (x - x0)^m_ \!$$\*UnderoverscriptBox[\(\[Sum]$$, $$n = 0$$, $$\[Infinity]$$]$$s_*\ \*SuperscriptBox[\((x - x0)$$, $$q_$$]\)\) -> \!$$\*UnderoverscriptBox[\(\[Sum]$$, $$n = 0$$, $$\[Infinity]$$]$$s*\ \*SuperscriptBox[\((x - x0)$$, $$q + m$$]\)\)


yielding

and the last one:

 expr5 = expr4 /. \!$$\*UnderoverscriptBox[\(\[Sum]$$, $$n = 0$$, $$\[Infinity]$$]$$u1_* v_$$\) + \!$$\*UnderoverscriptBox[\(\[Sum]$$, $$n = 0$$, $$\[Infinity]$$]$$u2_* v_$$\) -> \!$$\*UnderoverscriptBox[\(\[Sum]$$, $$n = 0$$, $$\[Infinity]$$]$$\((u1 + u2)$$*v\)\)


gives the result:

• The first and second pictures (yielded results), they look the same. Commented Mar 27, 2017 at 11:55