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I want to expand a very huge expression. Is there any way to do that without Series? Series works very slow, even when I want to find 

Series[expression,{x,0,0}]

I can not put x=0 and find the value of the expression because there are some terms like below in the expression:

...+ f/x-f/x +...

I also can not also use Simplify or Limit, because it is large and MM can not simplify it.

Can anyone help me please?

a simple example could be:

a^2/x+ x/b+ c x^2/+ g x^10/Sqrt[x^2+v^2] - a^2/x +...

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  • $\begingroup$ Perhaps Limit[expr,x->0] could help? What means f1/(x)-f1/(x)`? $\endgroup$
    – Ulrich Neumann
    Commented Jul 14, 2018 at 9:55
  • $\begingroup$ It is also very difficult for MM @UlrichNeumann $\endgroup$
    – Holger Mate
    Commented Jul 14, 2018 at 9:58
  • $\begingroup$ I have edited f1... @UlrichNeumann $\endgroup$
    – Holger Mate
    Commented Jul 14, 2018 at 9:58
  • $\begingroup$ Mathematica evaluates f/x - f/x (* 0*), so you probably mean somthing like a singularity ? Perhaps you can provide a simplified example of your problem? $\endgroup$
    – Ulrich Neumann
    Commented Jul 14, 2018 at 10:04
  • $\begingroup$ Yes. It is why I can not put x=0 and find what I am seeking for. @UlrichNeumann $\endgroup$
    – Holger Mate
    Commented Jul 14, 2018 at 10:05

1 Answer 1

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First, let us generate a large sum with terms similar to your mini-example.

hugeExpr[n_] := 
  Plus @@ Array[
    RandomInteger[{-10, 10}] x^
       RandomInteger[{-3, 1000}] Sqrt[x + RandomInteger[{-10, 10}]]^
       RandomInteger[{-1, 1}] &, n];

expression = hugeExpr[300000];

The actual number of addends (Length[expression]) might not be 300K, but it will still be a large count. Now, the answer to your problem.

ParallelMap[Series[#, {x, 0, 2}] &, expression, Method -> "ItemsPerEvaluation" -> 15]

I tried it on a expression with over 40K terms and it took less than 2 seconds. If your expression is orders of magnitude larger than that, I would suggest you split it in chunks of say 100K addends, use ParallelMap on each chunk separately, save those results to individual files, and add them up later.

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  • $\begingroup$ Thank you very much for your nice answer. When I try below code, MM still do not return anything: exp1 = f2x (-M11xx + M12xx); exp11 = ParallelMap[Series[#, {al, 0, 2}] &, exp1, Method -> "ItemsPerEvaluation" -> 15] // Normal; $\endgroup$
    – Holger Mate
    Commented Jul 15, 2018 at 14:18
  • $\begingroup$ exp1 is here: pastebin.com/RjJtAALb $\endgroup$
    – Holger Mate
    Commented Jul 15, 2018 at 14:18
  • $\begingroup$ Is it possible to use the same method of parallel computing for SeriesCoefficient? $\endgroup$
    – Holger Mate
    Commented Jul 16, 2018 at 5:45

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