MATLAB Central Newsreader - lu() returns permutations of the rows of L?

lu() returns permutations of the rows of L?

David Doria — Mon, 22 Sep 2008 21:30:20 -0400

Is there any way to have lu(A) give the actual lower triangular matrix L back? It seems silly to me to have to rearrange it?

Thanks,

Dave

Re: lu() returns permutations of the rows of L?

Walter Roberson — Mon, 22 Sep 2008 21:47:15 -0400

David Doria wrote:
> Is there any way to have lu(A) give the actual lower triangular matrix L back?
> It seems silly to me to have to rearrange it?

Why not just use the three-output or four-output versions of lu() ?
The two-output version of lu() is the only one that returns
the permuted lower triangular matrix.

Re: lu() returns permutations of the rows of L?

David Doria — Mon, 22 Sep 2008 22:06:02 -0400

ah ok, thanks

Re: lu() returns permutations of the rows of L?

Steven Lord — Tue, 23 Sep 2008 14:12:36 -0400

"David Doria" <daviddoria@gmail.com> wrote in message
news:gb92pc$lhl$1@fred.mathworks.com...
> Is there any way to have lu(A) give the actual lower triangular matrix L
> back? It seems silly to me to have to rearrange it?

As Walter said, call LU with three or more output arguments. To clarify:
when you call it with two outputs, you receive a '"psychologically lower
triangular matrix" (i.e. a product of lower triangular and permutation
matrices)' as the first output. The three-output form returns the two
matrices that you would have to multiply to get that psychologically lower
triangulat matrix as the first (the lower triangular matrix) and third (the
permutation matrix) outputs.

--
Steve Lord
slord@mathworks.com