How to realize the Robin Boundary condition on pde

[jsfeng-fudan]

I have one 1d ode :
-y' ' = 3 in domain x in [0,1], with the boundary
y'[0]-y[0] = -1 and y'[0]+y[0] = +1;
I tried to solve this problem using mfem library. but I can not arrive at correct solution.
The correct solution is

my solution is

I post my code for enforce the Robin boundary ..

    rbc_bdr = 1;
    fespace.GetEssentialTrueDofs(rbc_bdr, ess_tdof_list);
   
    double rbc_a_val1 = -1.0; // for left endpoint
    double rbc_b_val1 = -1.0; 
    double rbc_a_val2 = 1.0;  // for right endpoint
    double rbc_b_val2 = 1.0; 
    ConstantCoefficient rbc_a_coeff1(rbc_a_val1);// entering a matrix
    ConstantCoefficient rbc_a_coeff2(rbc_a_val2);// entering a matrix
    ConstantCoefficient rbc_b_coeff1(rbc_b_val1); // entering b linear form
    ConstantCoefficient rbc_b_coeff2(rbc_b_val2); // entering b linear form           
    b.AddDomainIntegrator(new DomainLFIntegrator(f_src));
    // key steps: add B.C. into linear system
    // Add Robin BCs n.(matCoef Grad u) + rbcACoef u = rbcBCoef on the boundary
    // // marked in the rbc_marker array.
    // left
    rbc_bdr = 0;
    rbc_bdr[0] =1 ;
    b.AddBoundaryIntegrator(new BoundaryLFIntegrator(rbc_b_coeff1), rbc_bdr);
    rbc_bdr = 0;
    rbc_bdr[1] =1 ;
    b.AddBoundaryIntegrator(new BoundaryLFIntegrator(rbc_b_coeff2), rbc_bdr);                                   
    b.Assemble();
    //getchar();
    a.AddDomainIntegrator( new DiffusionIntegrator(one));
    // // Add Robin BCs n.(matCoef grad u) + rbcACoef u = rbcBCoef on the boundary
    // // marked in the rbc_marker array.
    rbc_bdr = 0;
    rbc_bdr[0] = 1;
    a.AddBoundaryIntegrator(new MassIntegrator(rbc_a_coeff1), rbc_bdr);
    rbc_bdr = 0;
    rbc_bdr[1] = 1;
    a.AddBoundaryIntegrator(new MassIntegrator(rbc_a_coeff2), rbc_bdr);       
    cout << a.Size() << endl;
    a.Assemble();

[jsfeng-fudan]

sorry, Robin boundary condition reads :
y'[0]-y[0] = -1 and y'[1]+y[1] = +1;
Can someone help to tackle this problem.

[jamiebramwell]

As this problem only involves MFEM data structures, I suggest posting in their repository: https://github.com/mfem/mfem/