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MathGroup Archive 2002

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Re: Bug in Solve?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg36849] Re: [mg36834] Bug in Solve?
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Sun, 29 Sep 2002 02:55:11 -0400 (EDT)
  • References: <200209280834.EAA25013@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

David wrote:
> 
> Hi,
> 
> inside a program I need to solve this linear equation in terms of p1.
> However something odds happens. Sometimes the solution is computed and
> sometimes the result is empty [I mean no output...]. Is this a bug of the
> solve command or am I doing something wrong? The problem is robust to:
> changing name to the variables and other makeups..
> 
> Thank you very much,
> 
> David

That's the weirdest bug I've seen in weeks. As it happens, it's mine. At
least the inconsistent behavior, that is. I'll fix it, and maybe also
try to address the issue of how to handle approximate numbers in testing
subexpressions for zero.

I've excised your code and put in place a substantially smaller example
that I believe is responsible. The table will tend to give erratic
results.

zz = (-1.*x^7*(-1. + p - 7.*x^6 + p*x^6 + 6.*x^7)*
  (7.000000000000002 - 7.000000000000002*x + 14.000000000000004*x^6 -
  14.000000000000004*x^7 + 7.000000000000002*x^12 -
6.999999999999998*x^13))/
  (p^2*(1. + 0.9*x^6)^2*(1. + x^6)^5);

Table [Developer`ClearCache[]; Developer`ZeroQ[zz], {10}]

One workaround would be to use exact input, say by preprocessing with
Rationalize.


Daniel Lichtblau
Wolfram Research


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