# Re: [isabelle] ln on negative numbers

```That would probably also solve my problem.

```
As long as "(a*b) powr c = a powr c * b powr c" holds unconditionally, I am happy. And if I am not mistaken, that is the case iff "ln (a*b) = ln a + ln b" holds for all non-zero a,b.
```
Cheers,

Manuel

On 30/04/15 11:44, Larry Paulson wrote:
```
```For real r>0, the complex log (Ln) satisfies

Ln(-r) = ln(r) + pi*I

So I donât like extending the domain of ln using 0, as opposed to ln(|r|). This would extend the validity of

0 < ?z â Ln (complex_of_real ?z) = complex_of_real (ln ?z)

Thereâs still the question of ln(0). Note that Ln(z) is defined for all complex numbers except when z=0.

Larry Paulson

```
```On 29 Apr 2015, at 15:02, Manuel Eberl <eberlm at in.tum.de> wrote:

Hallo,

I am currently in the process of writing some automation for Landau symbols. One of the problems that I currently have is that I would like to rewrite things like

O(Îx. (c*x) powr p)

to

O(Îx. c powr p * x powr p)

However, for c < 0 this is simply not true (well, morally not true). If c < 0, and w.l.o.g. x > 0, we have

c powr p = (c*x) powr p = exp (p * THE u. False)

I therefore suggest to define the real logarithm as "ln x = (if x â 0 then 0 else THE u. exp u = x)". The practical implications of this should be small, and it would allow stating a lot of lemmas involving "powr" in a much simpler form and allow transformations such as the one above.

Cheers,

Manuel

```
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