Re: log expression not simplified

*To*: mathgroup at smc.vnet.net*Subject*: [mg115837] Re: log expression not simplified*From*: Praeceptor <gianluca.cruciani at poste.it>*Date*: Sat, 22 Jan 2011 03:22:24 -0500 (EST)*References*: <ih3rec$bhe$1@smc.vnet.net>

Thanks to everybody contributing to this thread. In particular to Andrzej who turned my attention to the use of complexity functions in mathematica's simplifying tools. On 18 Gen, 11:53, Andrzej Kozlowski <a... at mimuw.edu.pl> wrote: > [...] > However, it is hard to get it to simplify the expression to -1 because th= e only way to do so seems to involve temporarily increasing the default com= plexity. Thus, to get it to work we need a craftily designed custom complex= ity function, like, for example, this one: > > f[expr_] := -2 Count[expr, _Log, {0, Infinity}] + LeafCount[expr] > > This function "rewards" Simplify for expanding logs and, in this case, th= e "incentive" works: > > Simplify[Log[a/b]/Log[b/a], Assumptions -> {a > 0, b > a}, > ComplexityFunction -> f] > > -1 > > [...] > > Andrzej Kozlowski