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Groups > comp.soft-sys.math.maple > #253
| From | acer <maple@rogers.com> |
|---|---|
| Newsgroups | comp.soft-sys.math.maple |
| Subject | Re: LinearSolve, BackwardSubstitute(GaussianElimination) - different answers? |
| Date | 2011-09-25 20:35 -0700 |
| Organization | http://groups.google.com |
| Message-ID | <15947162.1966.1317008101833.JavaMail.geo-discussion-forums@yqjc18> (permalink) |
| References | <ea40abbb-3b88-4778-9381-983cb4019475@i28g2000yqn.googlegroups.com> |
In the `LinearSolve` case, debugging can reveal that only the first two entries of the third column are being passed to the external solver. The third entry of the third column (which is the RHS of the augmented system) is passed as the value zero. And so if the external solver happens to use the second and third rows of `a` (instead of, say, the first and second) then that incorrect solution is found.
In other words, LinearSolve is computing the solution to this problem instead:
restart:
with(LinearAlgebra):
a:=<<3100|6400|23610>,<250|360|0>>;
[3100 6400 23610]
a := [ ]
[ 250 360 0]
evalf(LinearSolve(a));
[-17.56115702]
[ ]
[ 12.19524793]
It looks like a coding error in LinearSolve, possibly made after computing the rank of `a` (which is 2, for your example).
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LinearSolve, BackwardSubstitute(GaussianElimination) - different answers? lasalle25 <betty.n.love@gmail.com> - 2011-09-25 17:46 -0700
Re: LinearSolve, BackwardSubstitute(GaussianElimination) - different answers? acer <maple@rogers.com> - 2011-09-25 20:35 -0700
Re: LinearSolve, BackwardSubstitute(GaussianElimination) - different answers? Peter Pein <petsie@dordos.net> - 2011-09-26 11:48 +0200
Re: LinearSolve, BackwardSubstitute(GaussianElimination) - different answers? Ray Vickson <RGVickson@shaw.ca> - 2011-11-04 15:03 -0700
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