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"Declaring" a vector for NDSolve
*To*: mathgroup at smc.vnet.net
*Subject*: [mg70734] "Declaring" a vector for NDSolve
*From*: Eric Poolman <emp35 at email.med.yale.edu>
*Date*: Wed, 25 Oct 2006 01:40:00 -0400 (EDT)
Hello,
I am trying to use Mathematica to solve a set of differential
equations in an epidemiological model (of sexually-transmitted disease.)
I have been using NDSolve on a system that consists of the following
two equations:
P'[t]=A.P[t],
P[0]={0.9, 0.1, ...}
Where P is a vector and A a matrix. Initial efforts with a simple A
went fine (Mathematica gives an error, "Part :: partw:: Part 2 of P
[t] does not exist," but it still produces a correct interpolating
function (verified by solving the simple system with each equation
written out separately.))
I am now complicating the matrix, A, by including in some of its
cells functions of P[t]. In particular, some of the terms in A will
include P[t][[1]] through P[t][[9]]. While P[t][[2]] and higher
terms are handled "correctly" (I get the error message, but it does
not interrupt the solving process), P[t][[1]] is not. P[t][[1]] is
immediately evaluated as t.
I have the following simplified formulation which shows the issue:
S = {T[t][[1]], T[t][[2]]}
ModelEqs = {T'[t] == S, T[0] == {0, 0}}
sol = NDSolve[ModelEqs, {T}, {t, 0, 1}]
T[1] /. sol
S Evaluates immediately to {t,T[t][[2]]}, which then leads to the
final line evaluating to {{0.5,0}}, rather than {{0,0}} as it should.
My naive thought is that I need to be able to declare T[t] as a
vector, so that T[t][[1]] is not immediately evaluated. I do not
know if that is a reasonable approach, or if that would work (or how
to do it.)
I am attempting to write out the program flexibly enough to handle
varying numbers of diseases (and thus varying dimensions for the
matrices), and would much prefer not to write out the equations if I
can do this generally. If worst came to worst, I suspect I could use
subscripts instead of indices and generate each equation, but the
matrix solution would be much more elegant (excepting this one issue.)
Thanks in advance; any guidance is appreciated.
Eric
--------------------------------
Eric Poolman, MD, MBA
Post-doctoral Fellow
Epidemiology of Microbial Diseases
Yale School of Medicine
60 College Street, Room 147
New Haven, CT 06520-8034
eric.poolman at yale.edu
203-589-8925 cell
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