plot the solution obtained from desolve_system

[Flavio Coelho]

Hi,

I am trying to plot the solutions the following system of ODEs

var('t')

A = function('A',t)

B = function('B',t)

de1 = diff(A,t) == -k1*A+k2*B

de2 = diff(B,t) == k1*A-k2*B

sol = desolve_system([de1,de2],[A,B],ics=[500,0])

show(sol)

solA, solB = sol[0].rhs(), sol[1].rhs()

plot((solA,solB),(t,0,50))

however the solution includes A(0) and B(0) as seen below:

[A(t)=111(6A(0)−5B(0))e(−1120t)+511A(0)+511B(0),B(t)=−111(6A(0)−5B(0))e(−1120t)+611A(0)+611B(0)]

How do specify the initial contidions so that my plot command above works? Now I am getting the following error:

verbose 0 (2424: plot.py, generate_plot_points) WARNING: When plotting,
failed to evaluate function at 200 points.
verbose 0 (2424: plot.py, generate_plot_points) Last error message:
'unable to simplify to float approximation'
verbose 0 (2424: plot.py, generate_plot_points) WARNING: When plotting,
failed to evaluate function at 200 points.
verbose 0 (2424: plot.py, generate_plot_points) Last error message:
'unable to simplify to float approximation'

[kcrisman]

On Wednesday, July 10, 2013 4:26:02 PM UTC-4, Flavio Coelho wrote:

Hi,

I am trying to plot the solutions the following system of ODEs

var('t')

A = function('A',t)

B = function('B',t)

de1 = diff(A,t) == -k1*A+k2*B

de2 = diff(B,t) == k1*A-k2*B

We're missing some info here.  If I make these variables, the commands below don't work at all, so they must be constants of some kind.  Can you give values?  (Or I can use sol = desolve_system([de1,de2],[A,B],ics=[500,0],ivar=t) as my command.)

 

sol = desolve_system([de1,de2],[A,B],ics=[500,0])

show(sol)

solA, solB = sol[0].rhs(), sol[1].rhs()

plot((solA,solB),(t,0,50))

however the solution includes A(0) and B(0) as seen below:

[A(t)=111(6A(0)−5B(0))e(−1120t)+511A(0)+511B(0),B(t)=−111(6A(0)−5B(0))e(−1120t)+611A(0)+611B(0)]

How do specify the initial contidions so that my plot command above works? Now I am getting the following error:

I'd try

sol = desolve_system([de1,de2],[A,B],ics=[0,500,0],ivar=t)

because t=0 is probably what you implicitly wanted.  An initial condition system doesn't necessarily start at t=0, just wherever your information is.

[kcrisman]

I've opened http://trac.sagemath.org/sage_trac/ticket/14877 for the potential confusion of having the "wrong number of initial conditions".