Thread Subject: log with syms variables?

if i do this

syms a b;
simplify(log(a*b))

i get log(a*b)

if i do this:

syms a b positive;
simplify(log(a*b))

i get log(a) + log(b)

The later is what I would like. Is it necessary to declare
the symbolic variables as 'positive'? Or am I doing
something else wrong?

Thanks,

Dave

Can someone at least confirm this is the way it works?

Thanks,

Dave

"David Doria" <daviddoria@gmail.com> wrote in message
<fpk2e6$4db$1@fred.mathworks.com>...
> if i do this
>
> syms a b;
> simplify(log(a*b))
>
> i get log(a*b)
>
> if i do this:
>
> syms a b positive;
> simplify(log(a*b))
>
> i get log(a) + log(b)
>
> The later is what I would like. Is it necessary to declare
> the symbolic variables as 'positive'? Or am I doing
> something else wrong?
>
> Thanks,
>
> Dave

On 2008-02-22 15:01:02 -0500, "David Doria" <daviddoria@gmail.com> said:
> Can someone at least confirm this is the way it works?

You pretty much answered your own question. If your example isn't
enough "proof" that you need to declare variables as positive to
simplify as you desire, consider the following:

log( (-1)*(-1) ) = log(1) = 0

but

log(-1) = i*pi

so

log(-1) + log(-1) = 2i*pi


>
> Thanks,
>
> Dave
>
> "David Doria" <daviddoria@gmail.com> wrote in message
> <fpk2e6$4db$1@fred.mathworks.com>...
>> if i do this
>>
>> syms a b;
>> simplify(log(a*b))
>>
>> i get log(a*b)
>>
>> if i do this:
>>
>> syms a b positive;
>> simplify(log(a*b))
>>
>> i get log(a) + log(b)
>>
>> The later is what I would like. Is it necessary to declare
>> the symbolic variables as 'positive'? Or am I doing
>> something else wrong?
>>
>> Thanks,
>>
>> Dave