thanks so much.I use worksheet. Maybe the problem is becouse I always use collect(%, diff(diff(B, x), x)); and I have read that the ditto operators % act on the expressions last evaluated in time, not space. So Maybe I have to recompute every time, and it is also slw when I recompute,maybe because ditto operators do not simply recall the results of past computations but instead reevaluate the results of these computations. How can I optimize my worksheet?I attach it to you. I remember you that I have to find all the times derivative of the dipendent variables (up to 4th order derivatives) in function of spatial derivatives. the differential equation are linear. I send you just the second derivatives(it is more readable),I used always the same way to find also the third and fourth derivative.

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How can I send the worksheet?if I paste and copy it change format. I tried all the tree imput format...

Thanks so much. sometimes when I use collect it gives me 5 rows where is written

[ canonicalCoordinates, ChangeSymmetry, CharacteristicQ, D_Dx, DeterminingPDE, `Ηk`, FromJet, InfinitesimalGenerator, Infinitesimals, InvariantSolutions, InvariantTransformation, Invariants, PDEplot, ReducedForm, SimilaritySolutions, SimilarityTransformation, SymmetryTest, SymmetryTransformation, TWSolutions, ToJet, build, casesplit, charstrip, dchange, dcoeffs, declare, diff_table, difforder, dpolyform, dsubs, mapde, separability, splitstrip, splitsys, undeclare ]

and the only way that I found to go ahead is to recompile with !!! and so it works! do you now why?

thanks

alberto