Thanks @dokken.

I was able to replicate the Singular Poisson problem in dolfinx using your help. The dolfin example is here.

It might be nice to include this example in the dolfinx tutorial. Either way, I’ll just post the code I was able to get working here (apologies if this already exists elsewhere, it just took me a bit to put the pieces together):

```
from mpi4py import MPI
from dolfinx import mesh,plot
from dolfinx.fem import FunctionSpace,Function,form
from dolfinx.fem.petsc import create_vector,assemble_matrix,assemble_vector
from ufl import inner,TrialFunction,TestFunction,ds,dx,grad,exp,sin,SpatialCoordinate
from petsc4py import PETSc
import pyvista
# Create mesh and define function space
domain = mesh.create_unit_square(MPI.COMM_WORLD, 20,20, mesh.CellType.quadrilateral)
V = FunctionSpace(domain, ("CG", 1))
u = TrialFunction(V)
v = TestFunction(V)
x = SpatialCoordinate(domain)
f = 10*exp(-(((x[0] - 0.5)**2) + (x[1] - 0.5)**2) / 0.02)
g = -sin(5*x[0])
a = inner(grad(u), grad(v))*dx
L = f*v*dx + g*v*ds
# Assemble system
A = assemble_matrix(form(a))
A.assemble()
b=create_vector(form(L))
with b.localForm() as b_loc:
b_loc.set(0)
assemble_vector(b,form(L))
# Solution Function
uh = Function(V)
# Create Krylov solver
solver = PETSc.KSP().create(A.getComm())
solver.setOperators(A)
# Create vector that spans the null space
nullspace = PETSc.NullSpace().create(constant=True,comm=MPI.COMM_WORLD)
A.setNullSpace(nullspace)
# orthogonalize b with respect to the nullspace ensures that
# b does not contain any component in the nullspace
nullspace.remove(b)
# Finally we are able to solve our linear system ::
solver.solve(b,uh.vector)
#plotting stuff
tdim = domain.topology.dim
topology, cell_types, geometry = plot.create_vtk_mesh(domain, tdim)
grid = pyvista.UnstructuredGrid(topology, cell_types, geometry)
plotter = pyvista.Plotter()
plotter.add_mesh(grid, show_edges=True,opacity=0.25)
u_topology, u_cell_types, u_geometry = plot.create_vtk_mesh(V)
u_grid = pyvista.UnstructuredGrid(u_topology, u_cell_types, u_geometry)
u_grid.point_data["u"] = uh.x.array.real
u_grid.set_active_scalars("u")
plotter.add_mesh(u_grid.warp_by_scalar("u",factor=.5), show_edges=True)
plotter.view_vector((-0.25,-1,0.5))
if not pyvista.OFF_SCREEN:
plotter.show()
```