Re: Converting system of equat
- To: mathgroup@smc.vnet.net
- Subject: [mg10257] Re: Converting system of equat
- From: "Herbert Gintis" <gintis@econs.umass.edu>
- Date: Sat, 3 Jan 1998 05:07:12 -0500
- Organization: University of Massachusetts
- References: <67l8pv$1mo@smc.vnet.net>
Times[x] is interpreted as Times[x,1], which just gives x. So there's your result. You could write foo[Plus[x_,y_]] := Times[x,y], solving the problem. You may have to set the attributes for foo to HoldAll so that the Plus is not evaluated. I'm not sure. -- Herbert Gintis Department of Economics Phone: 413-586-7756 University of Massachusetts Fax: 413-586-6014 Amherst, MA 01003 gintis@econs.umass.edu http://www-unix.oit.umass.edu/~gintis/ Ersek_Ted%PAX1A@mr.nawcad.navy.mil wrote in message <67l8pv$1mo@smc.vnet.net>... > >Tom James wrote: > ----------------------------------------- | >|I have, as a solution to a system of O.D.E.'s, a number of equations >|with unknowns A, B, C, etc. I would like Mathematica to write this >|system in matrix form, i.e., y=M.x, M is a matrix, x is the unknowns. >| > >OK. M is a matrix, x is/are the unknowns. What is y? > >Well maybe the following example answers the question. > > >In[1]:= >soln=DSolve[{z'[x]==-y[x], y'[x]== z[x]+1, y[0]==1, z[0]==0}, > {y[x], z[x]}, x] > > >Out[1]={{y[x] -> Cos[x] + Sin[x], z[x] -> -1 + Cos[x] - Sin[x]}} > > >In[2]:= >m={ y[x],z[x] } /.soln[[1]] > >Out[2]= >{Cos[x]+Sin[x], -1+Cos[x]-Sin[x]} > > > Ted Ersek > > > id CAA01525; Mon, 22 Dec 1997 02:54:29 -0500 > (5.67a/IDA-1.5 for <mathgroup@smc.vnet.net>); Mon, 22 Dec 1997 >08:56:56 +0100 From: Gianluca Gorni <gorni@dimi.uniud.it> To: To: mathgroup@smc.vnet.net >mathgroup@smc.vnet.net > > >Hello Mathusers! > >What about this as a way of changing the head of an expression from a to >b: > >In[1]:= foo[a[x__]]:=b[x]; > >It seems to work: > >In[2]:= foo[a[x1,x2]] > >Out[2]= b[x1, x2] > >But not if the head is Plus and I want to change it to Times: > >In[3]:= foo[Plus[x__]]:=Times[x]; > >In[4]:= foo[Plus[x1,x2]] > >Out[4]= x1 + x2 > >Anybody can explain this difference? > >Cheers, > Gianluca Gorni > > >%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% > >Gianluca Gorni >Universita` di Udine >Dipartimento di Matematica e Informatica via delle Scienze 208 >I-33100 Udine UD >Italy > >Ph.:(39) (432) 558422 Fax:(39) (432) 558499 >mailto:gorni@dimi.uniud.it >http://www.dimi.uniud.it > >