Question

I have some expression which I want to gather various linear terms (CxxP, CxyP, CyyP) from. Currently I am using:

aaa = Collect[ expr, {CxxP, CxyP, CyyP}]

but the result is not returned in the order I specify (i.e. I would like CxxP to appear first etc):

-(1 + a) CxyP (a GC) - CxxP (k^2 l^2 + GC)

The reason for wanting this ordering is so I can extract the specific terms I need. If there is an easier way to do this I would love to know, e.g.

In[] = ExtractTerm[aaa, CxxP]
Out[] = -(k^2 l^2 + GC)

Using CoefficientList doesn’t preserve the order either, and doesn’t pick up coefficients for terms that aren’t there i.e. I want to be explicitly told that there aren’t any CyyP terms.

Thanks, Ewan

Answer 1

If I’ve understood you right, you can make an ordered list of coefficients like this:

Coefficient[aaa,#]&/@{CxxP, CxyP, CyyP}