Problem with Times. Bug ????
- To: mathgroup at yoda.physics.unc.edu
- Subject: Problem with Times. Bug ????
- From: barenco at eldp.epfl.ch (ADRI)
- Date: Thu, 3 Jun 1993 17:10:53 +0200
I am trying to implement a fast non-commutative operation under
Mma to work with non-commutative algebras (see a previous post).
Among the things I tried, one was to redefine the Times operation
in order to make it non- commutative on a certain class of
objects, and to reimplement the commutativity for all other
standard objects for which Times is usually commutative. I know that this
is _highly_ discouraged by the authors of Mma. I tried various
implementations, and got contradictory or inconsistent results, I
tracked down the problem and found this :
In:= b a
Out= a b ! this is the standard order, Times is
! This removes the commutativity of Times
In:= b a
Out= b a ! OK, it doesn't commute anymore
In:= a_*(b_+c_):=a*b + a*c;
! This introduces the left ditributivity
! of Times on Plus
In:= 2 (1 + b a)
Out= 2 + b a
! the result is as expected.
In:= 2 (1 - b a)
Out= 2 - 2 a b
! ???? the expression In matches the pattern
! on the left handside of In, but in the
! replacement process Mma seems to have forgotten
! that Times is not commutative anymore !!!
Is this a bug ? Is it due to a specific implementation of Times
in the Kernel of Mma, so that Times does not tolerate
modifications of its attributes ?
Thanks for your answers,
Inst. for Micro- and Optoelectronics
Dept. of Physics
Swiss Fed. Inst. of Technology (EPFL)
CH-1015 Lausanne (Switzerland)
email: barenco at eldpa.epfl.ch
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