[Maxima-bugs] [maxima:bugs] #2596 error in matrix exponentiation

[Barton W. <wil...@us...>]

Thanks for this.  Here is an alternative that is, I think, much simpler.  Let's replace the spendy test `check-spectral-rep` with a check that the putative nilpotent is really a nilpotent.  I think the previous check was overkill.  

Doing so yields:

~~~

(%i85) xxx : spectral_rep(mat)$

(%i86) zzz : first(xxx).second(xxx)+third(xxx) - mat$

(%i87) zzz : rectform(zzz)$

(%i88) zzz : expand(float(zzz));

(%o88) matrix([-(3.885780586188048e-16*%i)-5.551115123125783e-17,
               1.1102230246251565e-16-1.3877787807814457e-16*%i,
               2.7755575615628914e-17],
              [2.220446049250313e-16*%i+5.377642775528102e-17,
               -(3.0531133177191805e-16*%i),-(8.881784197001252e-16*%i)],
              [1.1102230246251565e-16-6.938893903907228e-16*%i,
               6.938893903907228e-17-1.6653345369377348e-16*%i,
               5.0306980803327406e-17*%i-2.220446049250313e-16])
~~~


---

**[bugs:#2596] error in matrix exponentiation**

**Status:** closed
**Group:** None
**Labels:** matrixexp 
**Created:** Wed Jun 19, 2013 01:40 PM UTC by kcrisman
**Last Updated:** Thu Sep 03, 2026 04:15 PM UTC
**Owner:** nobody


This is a regression in 5.30.0 from 5.29.1:
```
(%i1) m: matrix([%i*%pi]);
(%o1)                             [ %i %pi ]
(%i6) matrixexp(m),keepfloat:true;

Unable to find the spectral representation
 -- an error. To debug this try: debugmode(true);
(%i7) matrixexp(m),keepfloat:false;

Unable to find the spectral representation
 -- an error. To debug this try: debugmode(true);
```

Point of info - Barton W. has a workaround:

This bug is due to circa January 2013 changes to matrix inversion. A workaround is to set ratmx to true:
```
Maxima branch_5_30_base_98_g29f9239_dirty http://maxima.sourceforge.net
using Lisp Clozure Common Lisp Version 1.9-r15764  (WindowsX8632)

(%i1) matrixexp(matrix([%i*%pi]));

Unable to find the spectral representation

(%i2) matrixexp(matrix([%i*%pi])), ratmx=true;
(%o2)                                 - 1
```


---

Sent from sourceforge.net because max...@li... is subscribed to https://sourceforge.net/p/maxima/bugs/

To unsubscribe from further messages, a project admin can change settings at https://sourceforge.net/p/maxima/admin/bugs/options.  Or, if this is a mailing list, you can unsubscribe from the mailing list.