questions about TS functions

[tsjb00]

Hi! I have some questions about TS functions.

1. I tried to use TSDefaultComputeJacobian to provide the Jacobian matrix. It worked fine with TS_BEULER, but when I tried to use TS_CN, it didn't work. The error message indicates that it is not supported for Crank-Nicholson. Is it true or I might do something wrong? 
2. Is there a specific PETSc function to evaluate Jacobian for TS_CN? Any example of using TS_CN that I can refer to?
3. I tried to solve a simple time evolving diffusion problem with TS, using TS_BEULER and non-linear solver. When I tried to use big time step, sever over-prediction is observed. I would appreciate any suggestion on the criteria of maximum time step allowed. If I really need to use big time steps, any tips to set up TS to improve the results?

Many thanks in advance!

BJ

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