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Re: minors and cofactors

From: Dirk Laurie
Subject: Re: minors and cofactors
Date: Fri, 18 Sep 1998 16:16:28 +0200 (SAT)

David D. Clark wrote:
> Are there octave commands for determining minors and cofactors without
> using the :  
> i.e. is there a command like minor(i,j) will find the minor associated
> with the ith row and jth column.  The cofactor is not as big an issue.
Pity, because the cofactor is the easy case:

  cofA = inv(A).'*det(A)

The minors are a little harder:

  [m,n] = size(A); k = ones(1,n); k(2:2:n) = -k(2:2:n); minA = (k'*k).*cofA
If you insist on not using the colon at all, there are tortuous ways
of generating the vector [1 -1 1 -1 ...] without it.

If you need cofactors of a singular or non-square matrix, the problem
is a good deal harder.  In fact, I can't offhand think of a fast way 
of doing it.


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