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st: RE: Re: on symmetry


From   "FEIVESON, ALAN H. (AL) (JSC-SK) (NASA)" <alan.h.feiveson@nasa.gov>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Re: on symmetry
Date   Wed, 11 Jun 2003 08:22:06 -0500

Kit - I don't think this works very well - consider

. matrix A = (10,4,2 \ 3,10,5 \ 1,6,10)

. matrix list A

A[3,3]
    c1  c2  c3
r1  10   4   2
r2   3  10   5
r3   1   6  10

. matrix D=A-A'

. matrix list D

D[3,3]
    r1  r2  r3
c1   0   1   1
c2  -1   0  -1
c3  -1   1   0

. di det(D)
0

Al Feiveson

-----Original Message-----
From: Christopher F Baum [mailto:baum@bc.edu]
Sent: Wednesday, June 11, 2003 6:06 AM
To: statalist@hsphsun2.harvard.edu
Subject: st: Re: on symmetry


On Wednesday, June 11, 2003, at 02:33 AM, David wrote:

> Instead, I'd suggest that Stata develop an internally created and 
> maintained
> property for matrices that indicates whether or not it is symmetric.

I ran into this issue a while back when developing outtable, which 
outputs a matrix as a lower triangle iff symmetric. I thought there was 
  a way to access the internal identifier by which Stata stores only the 
lower triangle for a matrix which it considers symmetric (e.g. a 
covariance or correlation matrix). After consulting with Stata's 
developers, no such luck (there is (or was) no user access to that 
flag). They suggested that I use the condition
det(mat - mat') to determine whether the matrix was numerically 
symmetric, which is the test employed by outtable.

Kit

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