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MathGroup Archive 2003

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LinearSolve - consumption time for square and rectangular matrices

  • To: mathgroup at smc.vnet.net
  • Subject: [mg44013] LinearSolve - consumption time for square and rectangular matrices
  • From: kujanp at fel.cvut.cz (Petr Kujan)
  • Date: Fri, 17 Oct 2003 05:14:44 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hello MathGroup,

I don't know about big difference in consumption time in LinearSolve[] 
with square/rectangular matrices.

I am using Mathematica 5.0, PC: Duron 750, W2k

Here is example.
(***************************
1.) very fast:
input matrices are square {200,200} and rectangular {200,10}

In[1]:=
*)

SeedRandom[1];
A = Table[Random[],{200},{200}];
B = Table[Random[],{200},{10}];
LinearSolve[A,B];//Timing

(*
Out[1]=
{0.03 Second,Null}

2.) lots slower:
input matrices are rectangular {201,200}, {201,10} - Overdetermined

In[2]:=
*)

SeedRandom[1];
A = Table[Random[],{201},{200}];
B = Table[Random[],{201},{10}];
LinearSolve[A,B];//Timing

(*
Out[2]=
{0.761 Second,Null}

In[3]:=
0.761/.03

Out[3]=
25.3667
***************************)

I expect more difficult computation, but not so difficult.

Where is problem? Exists some option Method for LinerSolve[] or another
trick?

Thank in advance for all your help.
Best regards,
    Petr.


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