Definition of an Identity Matrix

Date: 08/09/99 at 12:00:59
From: noor
Subject: Identity Matrix

Can someone please tell me what is the definition of an identity 
matrix? Which of the following would be an identity matrix?

  -1   2              1   -3              -2    2
   3  -4             -5    7               3   -4
                                          -5    6

Date: 08/10/99 at 01:28:11
From: Doctor Rob
Subject: Re: Identity Matrix

None of these is an identity matrix. An identity matrix must be a 
square. All the off-diagonal entries are zero. Finally, the entries on 
the main diagonal are all ones. For example,

   [1  0  0]
   [0  1  0]
   [0  0  1]

- Doctor Rob, The Math Forum
  http://mathforum.org/dr.math/