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Re: Problem with a system of equations describing an exposure to lead...
*To*: mathgroup at smc.vnet.net
*Subject*: [mg50398] Re: Problem with a system of equations describing an exposure to lead...
*From*: Paul Abbott <paul at physics.uwa.edu.au>
*Date*: Thu, 2 Sep 2004 04:34:22 -0400 (EDT)
*Organization*: The University of Western Australia
*References*: <ch1ke0$ic4$1@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
In article <ch1ke0$ic4$1 at smc.vnet.net>,
Richard Palmer <dickp at bellatlantic.net> wrote:
> I have a system of equations
>
> Eqns={u[t]==a bl[t-1],
> bo[t]==b bl[t-1]-c bo[t-1],
> bl[t]==(1-a-b) bl[t-1]-c bo[t-1],
> u[0]==0,
> bo[0]==0,
> bl[0]==pb}
>
> They are supposed to represent a system of what happens when a person
> suffers a one-time exposure to lead (pb). At any given time, the person
> will pass some portion of the lead that is in the blood and soft tissue
> (bl[t]) through the urine (u[t]) while some portion moves to the bone
> (bo[t]) and a small portion moves from the bone back into the blood. This
> is easily solved with RSolve.
Why are you modelling this as a difference equation and not as a
differential equation?
> However, I am concerned the solution may not
> be correct: I believe that for any time t, the amount of lead in the bone
> and blood less all lead passed to date through the urine should equal the
> level of exposure (pb).
If I understand you correctly, you expect that, for any time t,
bo[t] + bl[t] + u[t] == pb
It is easy to show that this requirement is inconsistent with your
system of equations, by turning the equations and initial conditions
into a set of replacement rules:
eqnrules =
{ u[t_] -> a bl[t-1],
bo[t_] -> b bl[t-1]-c bo[t-1],
bl[t_] -> (1-a-b) bl[t-1]-c bo[t-1]};
ics = {u[0] -> 0, bo[0] -> 0, bl[0] -> pb};
For t = 1, the equations are consistent (as you will see from the RSolve
solution for numerical values):
bo[t] + bl[t] + u[t] /. t -> 1 /. eqnrules /. ics // Simplify
However, for t = 2, two applications of the rules reveals the problem:
bo[t] + bl[t] + u[t] /. t -> 2 /. eqnrules /. ics // Simplify
% /. eqnrules /. ics // Simplify
The result is not equal to pb unless the constants are suitably
restricted.
> Taking the Rsolve solutions and making a table of
> u[t], bl[t], and bo[t] for times 0-5 will show the problem (substitute
> random fractions for a,b, and c and take pb to be 1). Is the mistake in the
> solution or in my formulation of the model?
Looks like a model formuation problem to me ...
Cheers,
Paul
--
Paul Abbott Phone: +61 8 9380 2734
School of Physics, M013 Fax: +61 8 9380 1014
The University of Western Australia (CRICOS Provider No 00126G)
35 Stirling Highway
Crawley WA 6009 mailto:paul at physics.uwa.edu.au
AUSTRALIA http://physics.uwa.edu.au/~paul
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