Subs behavior

[Paul Royik]

from sympy import *

a = symbols('a',positive=True)

(1/(sec(a)**2-1)**(3/2)).subs(sec(a)**2-1,tan(a)**2)

last statement should output something close to 1/tan^3(a), or abs(tan(a))/tan^4(a)

But instead it outputs something unrelated: abs(tan(a))/tan^2(a)

Is this a bug or I'm missing something?

[Chris Smith]

gotcha: 3/2 -> 0. Use S(3)/2 instead or Rational(3, 2)

[Paul Royik]

This is just an example.

In my program this is done as

expr = (sec(a)**2-1)**3

expr = 1/sympy.sqrt(expr)

In general, I have more complicated expressions, so don't know when to substitute.

[Aaron Meurer]

This might be a bug.

Aaron Meurer

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[Chris Smith]

Does this explain what you are seeing?

>>> var('a')

a

>>> expr = 1/sqrt((sec(a)**2-1)**3);expr

1/sqrt((sec(a)**2 - 1)**3)

>>> expr.subs(sec(a)**2 - 1, tan(a)**2)

1/sqrt(tan(a)**6)

>>> var('a', positive=True)

a

>>> (1/sqrt((sec(a)**2-1)**3)).subs(sec(a)**2 - 1, tan(a)**2)

Abs(tan(a))/tan(a)**4

[Paul Royik]

live.sympy.org gives Abs(tan(a))/tan(a)**2

[Aaron Meurer]

It looks like this works in master now (it gives
Abs(tan(x))/tan(x)**4). I bisected it to

commit 8b7642466e756f55bcbe7f6e8c108908f576e743
Author: zanzibar7 <tcr-p...@psu.edu>
Date: Sat Nov 1 19:28:56 2014 -0400

fixes Abs(x)**-3 bug

This was the pull request https://github.com/sympy/sympy/pull/8359.

Aaron Meurer

On Sat, Dec 13, 2014 at 4:16 AM, Paul Royik <distan...@gmail.com> wrote:
> live.sympy.org gives Abs(tan(a))/tan(a)**2

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