Re: Are some equations unsolvable?

*To*: mathgroup at smc.vnet.net*Subject*: [mg128072] Re: Are some equations unsolvable?*From*: "Alexander Elkins" <alexander_elkins at hotmail.com>*Date*: Thu, 13 Sep 2012 03:40:04 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <k2pc54$99t$1@smc.vnet.net>

One way to get a result is to rewrite f placing everything into Exp[] like so: In[1]:= FullSimplify[Log[(1/(sigma*Sqrt[2 Pi]))*Sqrt[(Pi/(-1/(2*sigma^2)))]]] Out[1]= 1/2 (Log[-sigma] - Log[sigma]) In[2]:= Block[{f, m, sigma}, f = Exp[((m/sigma^2)^2)/4*(-1/2*sigma^2) + (m^2/(2*sigma^2)) + 1/2 (Log[-sigma] - Log[sigma])]; Solve[f == 216, sigma]] (*Solve::ifun message*) Out[2]= {{sigma -> -(((-1)^(1/4) m)/( 2 Sqrt[1/3 (\[Pi] + I Log[46656])]))}, {sigma -> ((-1)^(1/4) m)/( 2 Sqrt[1/3 (\[Pi] + I Log[46656])])}} Note that Log[46656] == 2 Log[216]. Perhaps this helps... "Sergio Sergio" <zerge69 at gmail.com> wrote in message news:k2pc54$99t$1 at smc.vnet.net... > Hi, > This is what I have: > > f = (1/(sigma*Sqrt[2 Pi]))*Sqrt[(Pi/(-1/(2*sigma^2)))]* > Exp[((m/sigma^2)^2)/4*(-1/2*sigma^2) + (m^2/(2*sigma^2))] > > Solve[f == 216, sigma] > > And I get this message: "This system cannot be solved with the methods available to Solve" > > Is it because there is no way to isolate sigma? Or am I doing something wrong? > > Thanks >