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)
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- Delivered-to: l-mathgroup@wolfram.com
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- 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
>