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[✓] Transform an expression to a certain form?

GROUPS:

Hi, in my current project i evaluate large sums of the form:

enter image description here

in the most cases f(i,k,n)=0 so im left with expressions of the form

b*a[3]*a[4] + c*a[4]*a[3] + ...

what i want are expresions of the form like:

(b+c)*a[3]* a[4]

Sometimes Simplify or FullSimplify is doing the job sometimes not...

An example of what such an result look like is attached.

Not my plan was to go through the expression and find all components which are proportional to a[i]*a[k] and sum up the prefactors, but i cant figure out a nice way of doing that.

Does someone have an idea how to put the expression in the right form?

Many thanks in advance!!!

Attachments:
POSTED BY: Jonathan Prexl
Answer
4 months ago

enter image description here

POSTED BY: S M Blinder
Answer
4 months ago
e = (5 Sqrt[2/3] a^2 a[2] a[4])/(3 Sqrt[a^2 + g^2]) - (
   5 Sqrt[2/3] g^2 a[2] a[4])/(3 Sqrt[a^2 + g^2]) - (
   2 Sqrt[2/3] a g \[Pi] Sqrt[(a^2 + g^2) (4 g^2 + \[Pi]^2)]
     a[3] a[5])/((a^2 + g^2) (4 g^2 + \[Pi]^2)) - (a^2 a[6])/re - (
   g^2 a[6])/re - (\[Pi]^2 a[6])/(3 re) + (a a[1] a[7])/Sqrt[6] + (
   Sqrt[3/2] g \[Pi] a[1] a[8])/Sqrt[4 a^2 + 4 g^2 + \[Pi]^2] + (
   a a[7] a[9])/Sqrt[6] + (Sqrt[3/2] g \[Pi] a[8] a[9])/Sqrt[
   4 a^2 + 4 g^2 + \[Pi]^2];

Separate out the a variables (but not a itself) and use Collect:

avars = Cases[Variables[e], _a];
Collect[e, avars]

((5 Sqrt[2/3] a^2)/(3 Sqrt[a^2 + g^2]) - (5 Sqrt[2/3] g^2)/(
    3 Sqrt[a^2 + g^2])) a[2] a[4] - (
 2 Sqrt[2/3] a g \[Pi] Sqrt[(a^2 + g^2) (4 g^2 + \[Pi]^2)]
   a[3] a[5])/((a^2 + g^2) (4 g^2 + \[Pi]^2)) + (-(a^2/re) - g^2/
    re - \[Pi]^2/(3 re)) a[6] + 
 a[1] ((a a[7])/Sqrt[6] + (Sqrt[3/2] g \[Pi] a[8])/Sqrt[
    4 a^2 + 4 g^2 + \[Pi]^2]) + (a a[7] a[9])/Sqrt[6] + (
 Sqrt[3/2] g \[Pi] a[8] a[9])/Sqrt[4 a^2 + 4 g^2 + \[Pi]^2]
POSTED BY: Daniel Lichtblau
Answer
4 months ago

Many Thanks exactly what i was looking for!

POSTED BY: Jonathan Prexl
Answer
4 months ago

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