Re: Simplifying {0,0,0}.X.{0,0,0}

*To*: mathgroup at smc.vnet.net*Subject*: [mg74031] Re: Simplifying {0,0,0}.X.{0,0,0}*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Wed, 7 Mar 2007 03:07:04 -0500 (EST)*References*: <esjg5a$26q$1@smc.vnet.net>

Hi, X /: Dot[X, a : {0 ..}] := a will help. Regards Jens Will Robertson wrote: > Hello, > > In brief: Is there a way that I can specify that "X" is a matrix and > will resolve to zero when dot multiplied with a vector of zeros? > > I'm constructing a swathe of expressions that contain things like > (when expanded): {0,0,0}.X.{0,0,0}, where X is an arbitrary (well, > positive definite) square matrix of obvious size, and I'd like to be > able to have mathematica simplify that for me without much > intervention. > > My only option at the moment is to manually perform the replacements > {{0, 0, 0}.X._ -> 0, _.X.{0, 0, 0} -> 0} (the left and right vectors > won't always both be zeros), but this is tedious to perform for a > large number of expressions. I could also define the matrix X in terms > of its indices and end up with expressions in terms of x11, x12, ... , > x33, but I'd rather not do that. > > Any help would be greatly appreciated. > > Many thanks in advance, > Will Robertson > >