#7660: arithmetic with inequalities confusing
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Reporter: burcin | Owner: burcin
Type: defect | Status: new
Priority: major | Milestone: sage-4.3
Component: symbolics | Keywords:
Work_issues: | Author:
Upstream: N/A | Reviewer:
Merged: |
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From the following sage-devel thread:
http://groups.google.com/group/sage-devel/t/951d510c814f894f
Arithmetic with inequalities can be confusing, since Sage does nothing to
keep the inequality ``correct``. For example:
{{{
On Thu, 10 Dec 2009 00:37:10 -0800 (PST)
"[email protected]" <[email protected]> wrote:
> sage: f = x + 3 < y - 2
> sage: f*(-1)
> -x - 3 < -y + 2
}}}
It seems MMA doesn't apply any automatic simplification in this case:
{{{
On Thu, 10 Dec 2009 09:54:36 -0800
William Stein <[email protected]> wrote:
> Mathematica does something weird and formal:
>
> In[1]:= f := x+3 < y-2;
> In[3]:= f*(-1)
> Out[3]= -(3 + x < -2 + y)
}}}
Maple acts more intuitively, though the way ``formal products`` are
printed leaves something to be desired, IMHO:
{{{
On Thu, 10 Dec 2009 14:15:53 -0800
William Stein <[email protected]> wrote:
> Here is what Maple does:
>
> flat:release_notes wstein$ maple
> |\^/| Maple 13 (APPLE UNIVERSAL OSX)
> ._|\| |/|_. Copyright (c) Maplesoft, a division of Waterloo Maple
> Inc. 2009 \ MAPLE / All rights reserved. Maple is a trademark of
> <____ ____> Waterloo Maple Inc.
> | Type ? for help.
> > f := x < y;
> f := x < y
>
> > f*(-3);
> -3 y < -3 x
>
> > f*z;
> *(x < y, z)
>
> > f*a;
> *(x < y, a)
}}}
We should multiply both sides of the inequality only if the argument is a
real number (as opposed to a symbol with real domain), and invert the
relation when the argument is negative.
Note that GiNaC leaves everything formal, like MMA, by default:
{{{
ginsh - GiNaC Interactive Shell (ginac V1.5.3)
__, _______ Copyright (C) 1999-2009 Johannes Gutenberg University
Mainz,
(__) * | Germany. This is free software with ABSOLUTELY NO
WARRANTY.
._) i N a C | You are welcome to redistribute it under certain
conditions.
<-------------' For details type `warranty;'.
Type ?? for a list of help topics.
> f= x < y;
x<y
> f*-1;
-(x<y)
> f*-5;
-5*(x<y)
> f*-z;
-z*(x<y)
> f*z;
z*(x<y)
}}}
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/7660>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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