I see; the syntax should be solve(frm_tilde, w_sol, rhs, solver_parameters=...) i.e. "solve(A, x, b)" rather than "solve(A, b, x)". On 29 March 2016 at 16:09, George Ovchinnikov <lives9@gmail.com> wrote:

Dear All,

I have trilinear form a(w,u,v) and usually solve a(w,u,v) = l(v), for u with fixed w.

Here I have:

a = w dot(grad(u), grad(v)) * dx, and l(v) = f * v * dx.

Now, given u_tilde I want to solve for w, i.e. find w_tilde such, that a(w_tilde, u_tilde, v) = l(v), for any v in suitable subspace.

I'm trying the following:

mesh = UnitSquareMesh(size, size) V = FunctionSpace(mesh, "CG", 1)

f = Function(V) f.interpolate(Expression("1"))

u_tilde = Function(V) u_tilde.interpolate(Expression("1/(1+exp(-1*x[0])) * 1/(1+exp(-1*(1-x[0]))) *1/(1+exp(-1*x[1])) * 1/(1+exp(-1*(1-x[1])))" ))

w_tilde = TrialFunction(V) v = TestFunction(V)

a_tilde = (w_tilde * dot(grad(u_tilde), grad(v))) * dx frm_tilde = assemble( a_tilde )

L = f * v * dx rhs = assemble(L)

w_sol = Function(V) solve(frm_tilde, rhs, w_sol, solver_parameters={"ksp_type": "preonly", "pc_type": "lu"}) File("w_sol.pvd") << w_sol

It fails to converge: 140 r = self.ksp.getConvergedReason() 141 if r < 0: --> 142 raise RuntimeError("LinearSolver failed to converge after %d iterations with reason: %s", self.ksp.getIterationNumber(), solving_utils.KSPReasons[r])

KeyError: -11

I'm using direct method, not the iterative one, so what is happening here?

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