[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Axiom-developer] [#193 Symbolic values (without variables) are not orde
From: |
wyscc |
Subject: |
[Axiom-developer] [#193 Symbolic values (without variables) are not ordered properly in 'EXPR INT'] Two different orderings mixed up |
Date: |
Wed, 20 Jul 2005 17:48:32 -0500 |
Changes
http://page.axiom-developer.org/zope/mathaction/193SymbolicValuesWithoutVariablesAreNotOrderedProperlyInEXPRINT/diff
--
Hi Martin:
You already know the reasons and I think the title of this page is unfair. It
is clear that the meaning of the operator '<' in 'EXPR INT' (or any domain that
includes symbolic expressions like 'POLY INT') is not meant to be an ordering
of the ground domain (here 'INT'). In polynomial rings, '<' is a term-ordering
to facilitate polynomial arithmetic and variable elimination methods. If I read
'expr.spad' correctly, when 'R' is an integral domain, 'EXPR R' has 'FRAC
SMP(R, Kernel EXPR R)' as its representation ('Rep'). You can explain better
what that means than I (I would much appreciate your writing a page explaining
kernels and operators in Axiom). In any case, the ordering is inherited from
the 'Rep' and so I believe '%e' or 'exp(1)' is treated as a symbol. Indeed that
may be the reason we have:
\begin{axiom}
exp(1)^2 - exp(2)
(exp(1)^2 - exp(2) = 0)::Boolean
\end{axiom}
The sign of a polynomial expression is the sign of the coefficient of the
highest monomial term.
The way that '4-%e < 0' can be evaluated they way you intend is to substitute a
numerical value for '%e'. So users should be aware that in Axiom, symbolic
entries are not the same as their numerical counterpart. What they should do is:
\begin{axiom}
4.0 - %e < 0
\end{axiom}
or
\begin{axiom}
(4-%e)::(EXPR Float) < 0
\end{axiom}
(This is reminescent of FORTRAN: type promotion from INTEGER to REAL). In other
words, if one wants to test numerical inequalities involving symbolic
constants, one should work in 'EXPR Float', not 'EXPR Integer' since most
symbolic constants are not integers. To provide an automatic type promotion
from 'EXPR INT' to 'EXPR Float' would interfere with the ordering in polynomial
expressions, unless a symbolic constant can be distinguished from a polynomial
variable. We cannot overload the operator '<' for two distinct orderings in the
same domain. I think the suggestion I posted in #47 of a 'SymbolicFloat' domain
may be worth exploring. In 'SymbolicFloat', we can retain symbolic manipulation
of expresions with symbolic constants by using the term-ordering of the
underllying representation, and still use the numerical ordering of the domain
'SymbolicFloat'.
William
--
forwarded from http://page.axiom-developer.org/zope/mathaction/address@hidden
- [Axiom-developer] [#193 Symbolic values (without variables) are not ordered properly in 'EXPR INT'] Two different orderings mixed up,
wyscc <=