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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

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.


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