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## Re: [Info-gama] Gama

**From**: |
Ales Cepek |

**Subject**: |
Re: [Info-gama] Gama |

**Date**: |
Wed, 2 Jul 2008 14:06:40 +0200 |

**User-agent**: |
Mutt/1.5.13 (2006-08-11) |

On Wed, Jun 25, 2008 at 03:41:21PM -0400, O'Shea, Terry (MTO) wrote:
>* I have noticed that the 1.9.05 version has an "envelope" algorithm. Can*
>* you please tell me about the algorithm, how it works and where it would*
>* be useful?*
Hi Terry,
"envelope" in gama-local algorithms is a shorthand for "Cholesky
decomposition using envelope method for reducing bandwidth". Envelope
(or skyline matrix/storage) is a simple sparse matrix scheme based on
reordering so that non-zeroes are near main diagonal. Gama uses a
simple RCM reordering algorithm for reducing the bandwidth (one of
open Gama tasks is to implement better strategies). See also
http://en.wikipedia.org/wiki/Sparse_matrix
http://en.wikipedia.org/wiki/Skyline_matrix
http://en.wikipedia.org/wiki/Reverse_Cuthill-McKee_algorithm
In case of small geodetic networks you cannot gain much but with
linear systems with O(10^3) unknowns envelope algorithm (sparse matrix
solution) reduces solution time from an hour or more to seconds on my
notebook (adjustment of gps vectors networks in gama-g3).
So if you have input data resulting in long computation time, try envelope.
Ales

**[Info-gama] Gama**, *O'Shea, Terry (MTO)*, `2008/07/01`
**Re: [Info-gama] Gama**,
*Ales Cepek* **<=**