Talk:Triangular matrix

Inverses/products of triangular matrices

The article clearly states that products of upper triangular matrices are upper triangular, but it doesn't make the similar (and also true) claim about lower triangular matrices. Further, I only vaguely get the impression that the inverses of upper/lower triangular matrices remain upper/lower triangular. We should probably state these properties more directly, and perhaps clean up the article in general. --Rriegs 04:11, 5 May 2007 (UTC)

I've added a paragraph about triangular matricies preserving form. Tom Lougheed 02:32, 12 August 2007 (UTC)

Removed false claim

I deleted a line from the article falsely claimed that

"Indeed, we have

$\mathbf {L} ^{-1}={\begin{bmatrix}1&&&&&0\\&\ddots &&&&\\&&1&&&\\&&-l_{i+1,i}&\ddots &&\\&&\vdots &&\ddots &\\0&&-l_{n,i}&&&1\\\end{bmatrix}},$

i.e. the off-diagonal entries are replaced by their opposites."

Except for the first sub-diagonal, the inverse of an atomic lower triangluar is not quite as simple as reversing signs. Consider this counter example:

$\mathbf {L} ={\begin{bmatrix}1&&\\2&1&\\3&4&1\\\end{bmatrix}}$

Notice that ${\begin{bmatrix}1&&\\2&1&\\3&4&1\\\end{bmatrix}}{\begin{bmatrix}1&&\\-2&1&\\-3&-4&1\\\end{bmatrix}}={\begin{bmatrix}1&&\\0&1&\\-8&0&1\\\end{bmatrix}}\neq \mathbf {I}$

Tom Lougheed 01:17, 12 August 2007 (UTC)

going by the artcle's terminology, the matrix in your e.g. is not a "Gauss matrix". article only claims that formula holds when a matrix is Gauss. Mct mht 01:25, 12 August 2007 (UTC)

The claim is still false. Look at the counter example. Tom Lougheed 01:27, 12 August 2007 (UTC)

the article says Gauss matrix only have 1 non-zero column below the diagonal. probably you didn't see that. for those matrices the claim holds trivially. Mct mht 01:34, 12 August 2007 (UTC)

You are quite correct: reading through the article, the math typesetting looks like a general form lower triangular that's been normalized. Not good. I've extended the typesetting for the matrix $\mathbf {L} _{i}$ to show all the lower diagonal zeros, and have added a section heading "special forms" to separate the paragraph from the general section on triangular matricies. Tom Lougheed 02:32, 12 August 2007 (UTC)

Algebra of upper triangular matrices

Is there a standard notation for the algebra/ring of upper triangular matrices?--129.70.14.128 (talk) 23:09, 16 December 2007 (UTC)

Quasi-triangular matrices

In MATLAB and related programs I have seen references to 'quasi-upper-triangular' matrices, but I can't find a definition. Would someone please add a definition here? --Rinconsoleao (talk) 22:12, 28 February 2008 (UTC)

null matrix

i wanna know if a null matrix would be called an upper triangular or lower triangular. —Preceding unsigned comment added by 117.200.51.168 (talk) 07:29, 17 March 2009 (UTC)