Query regarding solution of linear system in deal.ii using linear solver

[Krish]

Hello,

I am solving a problem that has 30 degrees of freedom. The newton update solution obtained from deal.ii (using both Direct and CG solver) seems to be less accurate (differs after 4 significant digits)  from the newton update solution obtained from Matlab using the same tangent_matrix and system_rhs. I am getting the two following solutions from the same tangent_matrix and system_rhs (data files containing the tangent_matrix and system_rhs attached).

    Matlab solution                 deal.ii solution

                                     0                   0
  -0.003033422503555  -0.003033560000000
                                     0                   0
  -0.002439900952733  -0.002439990000000
                                      0                   0
  -0.002737307578376  -0.002737430000000
   0.001007226333711   0.001007240000000
  -0.001069740617723  -0.001069780000000
   0.001663965890031   0.001663970000000
  -0.002605547185476  -0.002605610000000
   0.001113032138953   0.001113040000000
  -0.001469379092482  -0.001469430000000
   0.000916401048149   0.000916380000000
                                      0                   0
   0.001674445763014   0.001674440000000
                                      0                   0
   0.001041269520982   0.001041260000000
  -0.000000000000000  -0.000000000000000
                                      0                   0
   0.003033422503555   0.003033560000000
                                     0                   0
   0.002439900952733   0.002439990000000
                                     0                   0
   0.002737307578376   0.002737430000000
   0.001007226333711   0.001007240000000
   0.001069740617723   0.001069780000000
   0.001663965890031   0.001663970000000
   0.002605547185476   0.002605610000000
   0.001113032138953   0.001113040000000
   0.001469379092482   0.001469430000000

I am trying to figure out what could be the reason behind it and how to get a more accurate solution.

Thank you in advance.

Krish

[Simon Sticko]

Hi.

From your attached files it looks like you have saved the matrix and rhs with quite few digits. Do you get the same result if you increase the number of digits you write to your std::ofstream object?

Compare:
https://en.cppreference.com/w/cpp/io/manip/setprecision

Best,
Simon

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[Krish]

Hello Simon,

Thank you very much for the suggestion. The Matlab and deal.ii solutions matched with each other after increasing the number of digits written to the std::ofstream object.

Thanks,

Krish