I'm using TS to solve a set of DAE, which originates from a one-dimensional problem. The grid points are uniformly distributed. For simplicity, the DMDA is not employed for discretization.
At first, only the residual function is prescribed through 'TSSetIFunction', and PETSC produces converged results. However, after providing hand-coded Jacobian through 'TSSetIJacobian', the internal SNES object fails (residual norm does not change), and TS reports 'DIVERGED_STEP_REJECTED'. I have tried to add the option '-snes_test_jacobian' to see if the hand-coded jacobian is somewhere wrong, but it shows '||J - Jfd||_F/||J||_F = 1.07488e-10, ||J - Jfd||_F = 2.14458e-07', indicating that the hand-coded jacobian is correct. Then, I added a monitor for the internal SNES object through 'SNESMonitorSet', in which the solution vector will be displayed at each iteration. It is interesting to find that, if the jacobian is not provided, meaning finite-difference is utilized for jacobian evaluation internally, the solution vector converges to steady solution and the SNES residual norm is reduced continuously. However, it turns out that, as long as the jacobian is provided, the solution vector will NEVER get changed! So the solution procedure stucked! This is quite strange! Hope to get some advice. PETSC version=3.14.6, program run in serial mode. Regards Yu Cang
