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My program has been running for two days. Is this normal?

Hui Sun, Peking University
Posted 11 years ago
POSTED BY: Hui Sun
10 Replies
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Solving 40 nonlinear algebraic equations can take considerable time.
POSTED BY: Daniel Lichtblau
Okkes Dulgerci
Okkes Dulgerci
Posted 11 years ago
POSTED BY: Okkes Dulgerci

Thanks for your advice. I am now trying to use NDSolve. It's indeed much faster than using DSolve. But I've encountered another problem. For 6 second-order ODEs with 12 boundary conditions, I can obtain the exact symbolic solutions in relatively less time by using DSolve. Then I plotted the first derivative of the symbolic solutions, from which I got the right graph. But when I used NDSolve to solve the same problem, I got nothing after I plotted the first derivative of the numerical solutions. Why so? Besides, the graph of the symbolic solutions is the same as the graph of the numerical solutions. Then why the graphs of the first derivative of the two solutions are totally different? Looking forward to your reply. Many thanks!!
POSTED BY: Hui Sun
Okkes Dulgerci
Okkes Dulgerci
Posted 11 years ago

Can you upload your code.. Never mind, I guess you asked your question with another name and is solved..
POSTED BY: Okkes Dulgerci

Yes. Sorry, my fault. Now I know how to ask questions. The most efficient way of asking for help is uploading the problematic code, rather than describe the problem with empty words. Anyway, many thanks for your help and patience! :-)
POSTED BY: Hui Sun

It's hard to help you without seeing the actual problem. It could be that you have a simple typo or syntax error. You say that MM7 can find all general solution in a snap. So why not find the general solution first and then solve for the unknown coefficients by substituting the boundary conditions? This probably can be done step by step.
POSTED BY: Kay Herbert
POSTED BY: Marvin Ray Burns A.G.S. (cum laude)

Thanks for the advice. I am already trying to solve a smaller unit cell with less ODEs and less boundary conditions.
POSTED BY: Hui Sun

Boundary value problems are much tougher than initial value problems. You may wish to try quasi-linearization, an iterative approach, rather than the built-in method, if you are looking for a numerical solution.

http://library.wolfram.com/infocenter/Conferences/368/
POSTED BY: Frank Kampas

Thanks for the advice, but I kind of want symbolic solutions.
POSTED BY: Hui Sun
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