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## Re: how to solve triangular systems

**From**: |
David Bateman |

**Subject**: |
Re: how to solve triangular systems |

**Date**: |
Sun, 05 Nov 2006 20:43:07 +0100 |

**User-agent**: |
Thunderbird 1.5.0.7 (X11/20060921) |

Federico Poloni wrote:
>* Hi everyone,*
>* sorry to bother this mailing list but I don't seem to find this*
>* anywhere in the docs.*
>* about operators \ and / :*
>* When the system matrix is diagonal or triangular does Octave solve it in*
>* "the smart way" (using O(n) and O(n^2) flops respectively)? Which other*
>* structures are recognized and supported by these operators (for example,*
>* matlab says it recognizes permuted-triangular systems)?*
If you are using octave 2.9.6 or later then yes octave does indeed solve
only the forward or backward substitution for a triangular matrix. It
detects upper, lower triangular matrices, permuted upper and lower
triangular matrices, and hermitian matrices and does the right thing..
See the matrix_type function for the list of matrix types for sparse and
full matrices that are currently handled automatically by octave 2.9
>* If the answer to the first question is no, then:*
>* Has Octave a "standard" way to solve triangular systems, or does one*
>* have to write back-substitution from scratch every time?*
If you aren't using 2.9.6 or later, then the octave-forge chol function
and triangular matrix type might help. These are available in older
versions of octave-forge.. Although if this is what you want, I'd
strongly recommend using octave 2.9.9 instead..
Regards
David