I posted this example and a comment on this to #7392, just in case. On Aug 17, 5:40 pm, Jason Grout <jason-s...@creativetrax.com> wrote: > On 8/17/10 6:20 AM, vgermrk wrote: > > > > > > > I discussed the following issue with<logix> �...@irc: > > > sage: rank(matrix(2,[1.5,1.75,-1.5,-1.75])) > > 2 > > > gives a wrong answer. > > > Multiplying with a full rank matrix also gives unexpected results: > > > sage: rank( matrix(2,2,[1,0,0,1]) * matrix(2,[1.5,1.75,-1.5,-1.75]) ) > > 2 > > sage: rank( matrix(2,2,[1,0,1,1]) * matrix(2,[1.5,1.75,-1.5,-1.75]) ) > > 1 > > > And even more serious: multiplying with another rank one matrix: > > > sage: rank( matrix(2,2,[1,0,1,0]) * matrix(2,[1.5,1.75,-1.5,-1.75]) ) > > 2 > > Right now, Sage's computations with numeric matrices are woefully > inadequate using the RR field (which is what is used above). People are > working on this right now by making an interface with ALGLIB. > > If you want to use numeric matrices to do computations right now, it's > best to use RDF or CDF; those usually call out to numpy, which does a > much more appropriate thing. However, in this case, we don't call out > to numpy. Here are related tickets from a while ago to fix these kinds > of bugs for RDF/RR: > > http://trac.sagemath.org/sage_trac/ticket/3162 > > http://trac.sagemath.org/sage_trac/ticket/7392 > > However, a more immediate thing to do would be to print out a warning. > This is done for some operations, but apparently not for rank. > > Also, hopefully soon, the interface to ALGLIB will be integrated into > Sage, and things will be fixed. Volunteers to help are welcome! > > Thanks, > > Jason
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