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# Appendix to Part I: More details on NCCollectOnVariables

As described in §, when performing computations, it is very helpful to distinguish between different representations of a polynomial equation. For example, the polynomials in (5.2) and (5.3) are equal, but (5.3) is much nicer, since it makes it clear that the polynomial depends on A, B, X+Y and Z rather than just on A, B, X, Y and Z. For this reason, it is helpful to have a way to compute such ``parenthesized'' representation of a polynomial. One might then be interested in computing all such ``parenthesized'' representation of a polynomial.

The key to computing ``parenthesized'' representations of a polynomial is the use of NCCollectOnVariables which has the NCAlgebra command NCCollect at its core. Before discussing the NCCollect command, we define a notion of homogeneous (noncommuting) polynomial.

Definition 9.1 Let p be a polynomial and V be a set of variables. p is homogeneous in V if for every v V, the number of occurences of v in each term of p is independent of the term.

Of course, x y z x + x x is homogeneous in {x,x}, x y z x + x z y is homogeneous in {y, z} and but x y z x + x z is not homogeneous in {x, x} (since the number of x is the first term is 1 and the number of x is the second term is 0).

When given a polynomial p and a set of variables V, NCCollect first writes the polynomial p as a sum of polynomials which are homogeneous in V. For each summand, NCCollect writes (to the extent possible) the polynomial into a ``parenthesized'' form using the rules

 c p v + c p v p = p v (c + c p) c p v p + c p v = p v (c p + c) c p v p + c v p = (c p + c) v p c v p + c p v p = (c + c p) v p c p v p + c p v p = p v (c p + c p) c p v p + c p v p = (c p + c p) v p c v + c v p = v (c + c p) c v + c p v = (c + c p) v

where v is a variable in V, c and c are scalars, and p , p and p are polynomials.

If none of the above rules apply to a V-homogeneous polynomial, then we say that its only collected form is trivial. If a polynomial is a sum of polynomials whose only collected form is trivial, then we say that this sums only collected form is trivial.

Helton
Wed Jul 3 10:27:42 PDT 1996