Thank you very much.

@Kitonum Thanks you.

@John Fredsted  Thank you.

@vv  Thank you.

@Kitonum Thank you.

@taro But getCoeff(f,a^2) is wrong.

@Kitonum Great, thank you very much.

@John Fredsted Great, thank you very much.

@John Fredsted But createDegree(L,3) do not return my result.

@John Fredsted Great, thanks you very much.

@John Fredsted Sorry Sir, I typed the wrong, sgm() is per() and I have edited.

For 

ds := [(a-b)^2, (a-c)^2, (a-c+b)^2, (2*a-b-c)^2]

then permUnique(ds) return [(a-b)^2, (a-c)^2, (a-c+b)^2, (2*a-b-c)^2] but it's wrong because (a-b)^2, (a-c)^2 in per((a-b)^2). The true result is [(a-c)^2, (a-c+b)^2, (2*a-b-c)^2]  or [(a-c)^2, (a-c+b)^2, (2*a-b-c)^2].

Thanks you very much.

@John Fredsted Thanks you sir, here's what i need but if

ds := [(a-b)^2, (a-c)^2, (a-c+b)^2, (2*a-b-c)^2]

then return [(a-b)^2, (a-c)^2, (a-c+b)^2, (2*a-b-c)^2] but (a-c)^2 in per((a-b)^2). How to improve it.

@Carl Love Thanks you.

@Kitonum Thanks you very much.