From: Gabor Grothendieck <ggrothendieck_at_gmail.com>

Date: Thu 01 Dec 2005 - 03:50:09 EST

Date: Thu 01 Dec 2005 - 03:50:09 EST

Sorry I seemed to have messed up the copying and pasting. Here it is again.

--- Go to http://mathomatic.orgserve.de/math/ and install mathomatic (its free) or just connect to the online server and do this. The C output, i.e the result of the two code c commands, can be used verbatim in R. Note that mathomatic does not support logs but for simple problems like this its very useful. Note that 1-> and 2-> are the mathomatic prompts and what comes after them are what I typed in. The entry goes into the corresponding equation space, i.e. equation 1 or equation 2. This is what you enter: mean = a/(a+b) variance = (a*b)/(((a+b)^2) * (a+b+1)) eliminate b a simplify code c eliminate a b simplify code c and this is the entire session: 1-> mean = a/(a+b) aReceived on Thu Dec 01 04:38:41 2005

#1: mean = -------

(a + b) 1-> variance = (a*b)/(((a+b)^2) * (a+b+1)) a*b

#2: variance = -------------------------

(((a + b)^2)*(a + b + 1)) 2-> eliminate b Solving equation #1 for (b)... 1 (a^2)*(---- - 1) mean

#2: variance = ---------------------------------------------------

1 1 (((a + (a*(---- - 1)))^2)*(a + (a*(---- - 1)) + 1)) mean mean 2-> a mean*(1 - mean)

#2: a = mean*(--------------- - 1)

variance 2-> simplify ((mean^2) - (mean^3))

#2: a = --------------------- - mean

variance 2-> code c a = ((((mean * mean) - pow(mean, 3.0)) / variance) - mean); 2-> eliminate a Solving equation #1 for (a)... b*mean ((mean^2) - (mean^3))

#2: ---------- = --------------------- - mean

(1 - mean) variance 2-> b mean*(1 - mean)

#2: b = (--------------- - 1)*(1 - mean)

variance 2-> simplify ((mean^2) - mean)

#2: b = (1 + -----------------)*(mean - 1)

variance 2-> code c b = ((1.0 + (((mean * mean) - mean) / variance)) * (mean - 1.0)); On 11/30/05, Gabor Grothendieck <ggrothendieck@gmail.com> wrote: > Go to http://mathomatic.orgserve.de/math/ and install mathomatic > (its free) or just connect to the online server and do this. > > The C output, i.e the result of the two code c commands, > can be used verbatim in R. > > Note that mathomatic does not support logs but for simply > problems like this its very useful. > > Note that 1-> and 2-> are the mathomatic prompts and what > comes after them are what I typed in. The entry goes into > the corresponding equation space, i.e. equation 1 or equation 2. > > 1-> mean = a/(a+b) > > a > #1: mean = ------- > (a + b) > > 1-> variance = (a*b)/(((a+b)^2) * (a+b+1)) > > a*b > #2: variance = ------------------------- > (((a + b)^2)*(a + b + 1)) > > 2-> eliminate b > Solving equation #1 for (b)... > > 1 > (a^2)*(---- - 1) > mean > #2: variance = --------------------------------------------------- > 1 1 > (((a + (a*(---- - 1)))^2)*(a + (a*(---- - 1)) + 1)) > mean mean > > 2-> a > > mean*(1 - mean) > #2: a = mean*(--------------- - 1) > variance > > 2-> simplify > > ((mean^2) - (mean^3)) > #2: a = --------------------- - mean > variance > > 2-> eliminate a > Solving equation #1 for (a)... > > b*mean ((mean^2) - (mean^3)) > #2: ---------- = --------------------- - mean > (1 - mean) variance > > 2-> b > > mean*(1 - mean) > #2: b = (--------------- - 1)*(1 - mean) > variance > 2-> simplify > > ((mean^2) - mean) > #2: b = (1 + -----------------)*(mean - 1) > variance > > > 2-> code c > b = ((1.0 + (((mean * mean) - mean) / variance)) * (mean - 1.0)); > > 2-> #1 > > b*mean > #1: a = ---------- > (1 - mean) > > 1-> code c > a = (b * mean / (1.0 - mean)); > > > > On 11/30/05, Scott Story <sstory@montana.edu> wrote: > > I am trying to write a function that will solve a simple system of > > nonlinear equations for the parameters that describe the beta > > distribution (a,b) given the mean and variance. > > > > > > mean = a/(a+b) > > variance = (a*b)/(((a+b)^2) * (a+b+1)) > > > > Any help as to where to start would be welcome. > > > > > > > > -- > > Scott Story > > Graduate Student > > MSU Ecology Department > > 319 Lewis Hall > > Bozeman, Mt 59717 > > 406.994.2670 > > sstory@montana.edu > > > > ______________________________________________ > > R-help@stat.math.ethz.ch mailing list > > https://stat.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html > > > ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html

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