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Topic: Handling branch cuts in trig functions
Replies: 9   Last Post: Mar 26, 2013 4:54 PM

 Messages: [ Previous | Next ]
 G. A. Edgar Posts: 2,510 Registered: 12/8/04
Re: Handling branch cuts in trig functions
Posted: Mar 25, 2013 8:52 AM

In article <kimoma\$hru\$1@speranza.aioe.org>, Nasser M. Abbasi
<nma@12000.org> wrote:

>
> But I am using Maple 17?
>
> -----------------------------------
> ans:=simplify(sqrt(sec(x)^2)) assuming x::positive;
>
> 1
> --------
> |cos(x)|
>
> simplify(abs(sec(x))- ans);
> 0
>
> -------------------------------------
>
> Unless x::positive implies x::real (since positive does
> not apply to complex numbers). Is this what you meant?

Yes, positive implies real. You will also get that result assuming x
is negative, or assuming x is an integer, and so on. Not only on the
reals, but also on any subset of the reals we have sqrt(x^2) = abs(x) .

>
> So Maxima was wrong then:
>
> sqrt(sec(x)^2);
> |sec(x)|
>
> No assumptions!

We cannot tell whether Maxima is wrong unless we know whether Maxima
assumes x is real (when you do not tell it). Maple assumes x is
complex, as was said. Perhaps the documentation for Maxima tells you

and the square-root of the square of that is itself, not its absolute
value. (Assuming principal branch.)

--
G. A. Edgar http://www.math.ohio-state.edu/~edgar/

Date Subject Author
3/24/13 Nasser Abbasi
3/24/13 G. A. Edgar
3/24/13 Nasser Abbasi
3/24/13 Richard Fateman
3/24/13 Axel Vogt
3/25/13 G. A. Edgar
3/26/13 Richard Fateman
3/26/13 Axel Vogt
3/24/13 clicliclic@freenet.de
3/24/13 Axel Vogt