Hi, try facsum:
(%i1) expr:+ i*y*a*r + k*w*a*r + b*w*y*r + b*i*k*r - w*y*a*t - i*k*a*t -
b*i*y*t + b*k*w*t;
(%o1) - a t w y + b r w y - b i t y + a i r y + b k t w + a k r w - a i k t
+ b i k r
(%i2) facsum(expr,r,[a,b],t,[a,b]);
(%o2) r (b (w y + i k) + a (i y + k w)) + t (- a (w y + i k) - b (i y - k w))
(%i3)
For more information you can type
demo("facexp");
> hi all,
>
> i have following expression
> (%i1) + i*y*a*r + k*w*a*r + b*w*y*r + b*i*k*r - w*y*a*t - i*k*a*t -
> b*i*y*t + b*k*w*t;
> (%o1) - a t w y + b r w y - b i t y + a i r y + b k t w + a k r w - a
> i k t + b i k r
>
> then i can do
> (%i2) collectterms(%,r,t);
> (%o2) r (b w y + a i y + a k w + b i k) + t (- a w y - b i y + b k w - a i k)
>
> then i would like to do
> (%i3) some magic to get
> (%o3) r (b (w y + i k) + a (i y + k w )) + t (- a( w y + i k ) - b( i y - k w))
> i.e. i would like to apply collectterms second time to particular
> parenthesed expressions.
>
> is it somehow possible? the method should be general for variable
> number of parenthesed expressions in second level. i appreciate any
> help.
>
> thanks in advance
> vit mach-zizka
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>
>