[petsc-users] SNES solvers

[Abdul-Lateef Haji-Ali]

Greetings,

I am trying to solve a system of 4 non-linear equations to find 4 unknowns.
The equations are slightly complicated  and unstable for slightly large
values of the unknowns.
In most of the cases the correct solution is found. However sometimes the
values that are fed to my residual function are big and nan/inf values are
produced.

This is the output of my residual function (x are the parameters and f is
the computed residual)
x= [1, 1, 2, 4]; f= [-1.32993, 6.46116, 0.490443, -4.47854];
x= [2.32993, -5.46116, 1.50956, 8.47854]; f= [-1.77078, -133.872, 4.26217,
71.9728];
x= [1.06488, 0.684771, 1.97607, 4.2185]; f= [-0.431978, 2.79625, 0.190123,
0.0503181];
x= [4.10071, 128.411, -2.75261, -63.4942]; f= [62.0141, 2548.28, -95.0523,
-1354.74];
x= [1.74294, 0.960827, 1.63558, 4.54888]; f= [19.6641, 4.52508, -9.48464,
7.44219];
x= [-57.9134, -2419.86, 92.2997, 1291.24]; f= [inf, -inf, inf, -inf];
This was using ngmres, I noticed similar behaviour with other types.

What can I do to improve the solver?
N
ote that my experience with petsc in new and limited.

Thank you,

-- 
Abdul Lateef Haji Ali
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