Re: Working with Log

*To*: mathgroup at smc.vnet.net*Subject*: [mg112113] Re: Working with Log*From*: Patrick Scheibe <pscheibe at trm.uni-leipzig.de>*Date*: Tue, 31 Aug 2010 04:16:17 -0400 (EDT)

Hi, there's no unexpand but to know that PowerExpand uses heavy assumptions per default is useful. This should be a warning too that the results of PowerExpand not in general true. See for instance what PowerExpand[Log[a b], Assumptions -> Element[{a, b}, Complexes]] gives: 2*I*Pi*Floor[1/2 - Arg[a]/(2*Pi) - Arg[b]/(2*Pi)] + Log[a] + Log[b] Your expansion is valid for instance for PowerExpand[Log[a b], Assumptions -> a > 0 && b > 0] and therefore FullSimplify[%, Assumptions -> a > 0 && b > 0] gives again Log[a b] To see where exactly the log rule works you could do maybe Reduce[2*I*Pi*Floor[1/2 - Arg[a]/(2*Pi) - Arg[b]/(2*Pi)] == 0, {a, b}] Cheers Patrick On Mon, 2010-08-30 at 06:20 -0400, Themis Matsoukas wrote: > 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 >