Strange behaviour of "expand" in CAS view (Xcas problem?)

10162014, 02:29 PM
Post: #1




Strange behaviour of "expand" in CAS view (Xcas problem?)
Typing
Code: expand(x*2.9+6.3) Code: 2.9*x+6.3 Typing Code: expand(x*2.9e13+6.3e13) Code: (1+x)*6.3e13 This happens when both powers of ten are 13 or smaller. This also happens in Xcas on my PC (in the latest stable and unstable versions). Nigel (UK) 

10162014, 05:13 PM
Post: #2




RE: Strange behaviour of "expand" in CAS view (Xcas problem?)
Probably has to do with how you set up the CAS to interpret small numbers. If those are floating point numbers, your settings may have forced the CAS to assume them to be 0.
Graph 3D  QPI  SolveSys 

10162014, 08:06 PM
Post: #3




RE: Strange behaviour of "expand" in CAS view (Xcas problem?)
I can't reproduce on Xcas
0>> expand(x*2.9e13+6.3e13) 2.9e13*x+6.3e13 The latest stable version is 1.1.210 (version() on windows and mac, dpkg l giac on linux to check your version). 

10192014, 07:14 PM
Post: #4




RE: Strange behaviour of "expand" in CAS view (Xcas problem?)
(10162014 08:06 PM)parisse Wrote: I can't reproduce on Xcas Sorry about the delay in my reply  real life intervened! I have reinstalled Xcas  version() now returns Code: "giac 1.1.210, (c) B. Parisse and R. De Graeve, Institut Fourier, Universite de Grenoble I" Code: exact real DEG 10 xcas I still have the error described above. It may be to do with the way in which Xcas treats small numbers, as Han suggests. However in this case Xcas isn't replacing one small number by zero; rather, it is replacing one small number by the other small number in the problem. This is unexpected. 

10202014, 07:18 AM
Post: #5




RE: Strange behaviour of "expand" in CAS view (Xcas problem?)
Ok, I see. I was testing with the commandline version (giac) not with Xcas, because the commandline version is more suited to debugging.
I will modify expand not to collect if the coefficients are not exactly the same (currently it is comparing the difference to 0, and epsilon is used for doubles, but it's the null context epsilon, not updated by modifying epsilon in the settings). 

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