There are a few problems

Joe Riel — Sat, 13 Mar 2010 09:11:13 Z

There are a few problems with this. First, the if functions are evaluated immediately. Second, the conditions of the if's are expressed as equalities, which will not work. The independent variable (t) is continuous, as such it makes no sense numerically to compare it to any value. You should use inequalities (t<=0, t<=1, t<=2, t<=3). Third, the differential equation has the parameters b, c, and n, which are undefined. Probably they are meant to be functions of e and f.

You can express the if's using the piecewise function. Here is one approach; I arbitrarily reassigned b, c, and n:

a := .178131:

pw := piecewise(t <= 1, 6, t <= 2, 10, t <= 3, 15, 20):

e := -0.6814e-1*pw:
f := 0.16242e-1*pw^(1.5):

# Arbitrary assignments
n := 2:
b := e:
c := f:

dsys := [diff(Log10S(t), t) = -(a+b+c)*n*(-Log10S(t)/(a+b+c))^((n-1)/n), Log10S(0) = -0.1e-3]:
integ := dsolve(dsys, numeric):
integ(6);
                    [t = 6., Log10S(t) = -4.28489796949036]

Solving an ODE with `if`statement 2

cactus — Sat, 13 Mar 2010 10:50:50 Z

Hi Joe:

Thank you so much for your help.

You came with an elegant solution, I appreciate it very much.

Moshe.

MaplePrimes - answers and comments on Question, Solving an ODE with `if`statement

There are a few problems

Solving an ODE with `if`statement 2