       Re: Simplify UnitStep expressions

• To: mathgroup at smc.vnet.net
• Subject: [mg69312] Re: [mg69195] Simplify UnitStep expressions
• From: Adam Strzebonski <adams at wolfram.com>
• Date: Wed, 6 Sep 2006 04:28:42 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Andrzej Kozlowski wrote:
>
> On 1 Sep 2006, at 11:41, L. Dwynn Lafleur wrote:
>
>> The following is transcribed from a Mathematica 5.2 notebook in
>> Windows XP:
>>
>> In:= Simplify[UnitStep[a-x/b], a-x/b > 0]
>> Out= 1
>>
>> In:= Simplify[UnitStep[a-Pi/b], a-Pi/b > 0]
>> Out= UnitStep[a-Pi/b]
>>
>> Why does the second output different from the first?  I know it has
>> something to do with the fact that Pi is internally defined in
>> Mathematica
>> because a similar result occurs Pi is replaced with E, but what  logic is
>> being followed?
>>
>> --
>> ======================================
>>  L. Dwynn Lafleur
>>  Professor of Physics
>>  University of Louisiana at Lafayette
>>  lafleur at louisiana.edu
>> ======================================
>>
>
> Curiously, if you use FullSimplify rather then Simplify you will get:
>
>
> FullSimplify[UnitStep[a-Pi/b], a-Pi/b > 0]
>
> 1
>
>
> The same holds if Pi is replaces by E, or indeed by explicit  functions
> of E or Pi such as Pi^2, E^Pi etc. In all such cases  FullSimplify works
> but Simplify does not work. Strange.
>
> Andrzej Kozlowski
>
>

The cylindrical algebraic decomposition (CAD) algorithm used by Simplify
to prove inference requires polynomial inequalities with rational number
coefficients. a-x/b > 0 is equivalent to a polynomial inequality
-(a*b^2) + b*x < 0 which has rational number coefficients.
a-Pi/b > 0 is equivalent to a polynomial inequality -(a*b^2) + b*Pi < 0
which has a numeric coefficient Pi which is not a rational number.

Mathematica has two ways of dealing with nonrational numeric
coefficients in CAD. One is to replace each nonrational coefficient
with a new variable. This method always allows to decide inference
(modulo the ability to zero-test the exact numeric constants), but
it is potentially very expensive - CAD has a doubly exponential
complexity in the number of variables and we add a new variable for
each nonrational coefficient. The second method replaces nonrational
numeric coefficients with their approximations. This is much less
expensive, but in some cases it fails to decide inference.
Simplify uses the second method which in this case is insufficient.

FullSimplify uses more transformations, and one of the additional
transformations succeeds.

Best Regards,