# Cas bugs in the evaluation of the logarithm?

1. Sep 30, 2009

### ilario980

hi,

can you evaluate $$ln(f(x))$$ in your favorite cas (where x is a zero of f(x) )? what is the result?

Last edited: Sep 30, 2009
2. Sep 30, 2009

### tiny-tim

Welcome to PF!

Hi ilario980! Welcome to PF!
If f(x) < 0, then ln(f(x)) isn't defined.

Other than that, the limit of ln(f(x)) will be -∞.

3. Sep 30, 2009

### ilario980

yes, i know; but what seems to me strange is that i tryed in some cas to evaluate that value and the ouput is not -inf and is different depending on the function f(x)

after this experience i believe that trust cas may be dangerous...

I.M.

4. Sep 30, 2009

### tiny-tim

Hi I.M.!

(btw, it's case, not cas )

Did you evaluate it using a calculator?

When you get near 0 or ∞, a calculator can give rounding errors that make a lot of difference!

5. Sep 30, 2009

### chroot

Staff Emeritus
He means "computer algebra system," or CAS.

- Warren

6. Sep 30, 2009

### tiny-tim

ah!

thanks, Warren!

7. Oct 1, 2009

### ilario980

hi Tiny-Tim,
i was not playing with my calculator keys
i was interpreting (superficially, i admit) some data and this behavior was leading me to an erroneous conclusion

Ilario M.

8. Oct 1, 2009

### Staff: Mentor

Another reason to not trust computers too much.