Re: A Problem with Simplify
- To: mathgroup at smc.vnet.net
- Subject: [mg87515] Re: A Problem with Simplify
- From: Alexey Popkov <popkov at gmail.com>
- Date: Sat, 12 Apr 2008 06:56:50 -0400 (EDT)
- References: <email@example.com> <firstname.lastname@example.org>
On 11 =C1=D0=D2, 13:59, Jean-Marc Gulliet <jeanmarc.gull... at gmail.com> wrote: > Alexey Popkov wrote: > > <snip> > > > Recently I also have found another strange bug in Simplfy (not so > > dangerous): > > FullSimplify[Sin[x]*Cos[x]] > > gives Sin[x]*Cos[x] instead Sin[2*x]/2. The workaround is > > FullSimplify[2*Sin[x]*Cos[x]]/2. > > <snip> > > This is not a bug, indeed. As a rule of thumb, *FullSimplify* tries to > minimize the leaf count of the expression. > > For the case above, the original expression sin(x)cos(x) has a leaf > count of 5 whereas the trigonometric identity sin(2x)/2 has a leaf count > of 8. Therefore, *FullSimplify* returns the original form sin(x)cos(x) > since it is deemed as simpler than sin(2x)/2 w.r.t. the lead count of > each expression. > > Note that, in general, you can modify/tweak this default behavior to > suit your needs by witting your own *ComplexityFunction*. However, when > dealing with trigonometric expressions (especially when looking for some > trigonometric identities) it is usually better and less cumbersome to > use *TrigReduce* and the like (*TrigExpand*, *TrigFactor*, etc.). > > FullSimplify[Sin[x]*Cos[x]] (* returns Cos[x] Sin= [x] *) > % // LeafCount (* returns = 5 *) > TrigReduce[Sin[x]*Cos[x]] (* returns 1/2 Sin[= 2 x] *) > % // LeafCount (* returns = 8 *) > LeafCount[Cosh[x] - Sinh[x]] (* returns 7 = *) > FullSimplify[Cosh[x] - Sinh[x]] (* returns E^-x = *) > % // LeafCount (* returns = 5 *) > > Best regards, > -- Jean-Marc Big thanks! I did not understand this from the documentation. But now this become clear.