@mehdi jafari 
Thanks for your reply.
Shouldn't be the output of tdeg and plex be different?
The maple documentation says, that plex(x,y,z) uses the lexicographical ordering (thus x>y>z) and should give x+y+z,
while tdeg(x,y,z) uses the total degree reverse lexicographical ordering (z>y>x). Since the test-polynomial x+y+z is homogenous, if would expect the output to be z+y+x.

perfect, thanks ! Almost; now i get the error "Error, (in gcd/LinZip) variable in input expression must be in either the evaluate or retained variables"

When i include "with(LinearAlgebra)" in the sheet it works fine.

perfect, thanks ! Almost; now i get the error "Error, (in gcd/LinZip) variable in input expression must be in either the evaluate or retained variables"

When i include "with(LinearAlgebra)" in the sheet it works fine.

for this approach to work, you need to guess/know alpha=\sqrt(-1) in advance. for more complicated ideals, i can not do that. Is there a way to let maple find the smallest 'decomposition field' necassary?

for this approach to work, you need to guess/know alpha=\sqrt(-1) in advance. for more complicated ideals, i can not do that. Is there a way to let maple find the smallest 'decomposition field' necassary?

The combination of convert and coeff works perfectly. Thanks a lot.

Of course i knew there are infinetly many coefficients, I just wanted a list of all coefficients (of a linear differential operator) up to a certain degree.

The combination of convert and coeff works perfectly. Thanks a lot.

Of course i knew there are infinetly many coefficients, I just wanted a list of all coefficients (of a linear differential operator) up to a certain degree.