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MathGroup Archive 2004

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RE: how can I solve a function Erfc

  • To: mathgroup at smc.vnet.net
  • Subject: [mg48428] RE: [mg48396] how can I solve a function Erfc
  • From: "David Park" <djmp at earthlink.net>
  • Date: Sat, 29 May 2004 03:06:50 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Aude,

How about something like this...

f[L_][t_] := Erfc[L/(2000*Sqrt[2]*Sqrt[t])]

You can look for roots by plotting the function, adjusting the iterator
domain for various values of L.

Plot[f[2000][t], {t, 0, 200},
    PlotRange -> {0.8, 1.0},
    ImageSize -> 500];

You can find a specific root, say for L == 2000 with...

FindRoot[f[2000][t] == 0.9, {t, 1}]
{t -> 63.3281}

Writing a general function g to return t for a given L...

g[L_] := t /. FindRoot[f[L][t] == 0.9, {t, Sqrt[L]}]

You can then plot the solution...

Plot[g[L], {L, 0.01, 10^4},
    ImageSize -> 500];

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/







From: aude [mailto:montgermont.aude at ec-lille.fr]
To: mathgroup at smc.vnet.net


Hi,

I have to solve this function:

Erfc[L/(4*(Dg*t)^(1/2))]= 0.9

Dg is constant.

Dg=5*10^5

I want to plot t as a function of L.

thanks.

aude




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