Re: solutions that are not solutions

*To*: mathgroup at smc.vnet.net*Subject*: [mg32073] Re: [mg32049] solutions that are not solutions*From*: KCConnolly at aol.com*Date*: Sat, 22 Dec 2001 04:22:45 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

psino at tee.gr wrote: >I'm trying to solve a system as follows: >X={{0,y,z},{y,x,t},{u,v,w}} >A={{1,1,a},{0,1,0},{0,0,1}} >Solve[{X.A==Transpose[A].X, Det[X]==1}, >{x,y,z,t,u,v,w}] >Mathematica 4.1 gives two solutions: >X1={{0,y,0},{y,x,t},{0,v,-1/y^2}} >and >X2={{0,y,a*y},{y,(a*t*y^2-1+a*v*y^2-w*y^2)/(a*y)^2,t}, >{a*y,v,w}} > >However, X1 is not a solution: >X1.A-Transpose[A].X1={{0,0,0},{0,0,a*y},{0,-a*y,0}} > >Could anybody explain this behaviour? I am still laboring with Mathematica 3.0, which only generates X2. However, X1 appears to be a re-expression of X2 in the instance where a=0, in which case the second entry in the second row of X2 would be undefined through having 0 in the denominator. And if a=0, then of course a*y and -a*y are 0, and so X1 is (in that limited instance) a solution. So X2 is the general solution, and X1 is the solution when X2 is undefined. Hope that helps. Kevin Connolly