MATLAB Central Newsreader - log with syms variables?

log with syms variables?

David Doria — Thu, 21 Feb 2008 14:39:02 -0500

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

Re: log with syms variables?

David Doria — Fri, 22 Feb 2008 20:01:02 -0500

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

Re: log with syms variables?

Arthur G — Sat, 23 Feb 2008 00:42:44 -0500

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