Re: Working with Log

*To*: mathgroup at smc.vnet.net*Subject*: [mg112101] Re: Working with Log*From*: Peter Breitfeld <phbrf at t-online.de>*Date*: Tue, 31 Aug 2010 04:14:06 -0400 (EDT)*References*: <i5g0ku$grr$1@smc.vnet.net>

Themis Matsoukas wrote: PowerExpand treats its expressions assuming all variables are positive. Normally Log[ab]==Log[a]+Log[b] is not always tru, e.g. 0=Log[(-1)(-1)]!=Log[-1]+Log[-1]==2Pi I So you have to declare the variables as positives (if your domains are right): In[1]:= Simplify[Log[a]+Log[b], Assumptions->a>0&&b>0] Out[1]= Log[ab] > I can use PowerExapnd to expand a Log: > > PowerExpand[Log[a b]] > > Log[a] + Log[b] > > How can I do the opposite, i.e. combine Log[a] + Log[b] into Log[a b]? > > Thanks > > Themis > -- _________________________________________________________________ Peter Breitfeld, Bad Saulgau, Germany -- http://www.pBreitfeld.de