Dear All, I was working on Helmholtz equation solver and found, that DG projection produces errors on periodic boundaries (XML file attached). Check the following log: ======================================================================= EquationType: Helmholtz Session Name: Helmholtz2D Spatial Dim.: 2 Max SEM Exp. Order: 7 Expansion Dim.: 2 Advection Type: Projection Type: Continuous Galerkin Lambda: 0 Forcing func [0]: -(Lambda + 2*alpha*alpha)*sin(alpha*x)*sin(alpha*y) ======================================================================= Writing: "Helmholtz2D.fld" (0.0222731s, XML) ------------------------------------------- Total Computation Time = 4s ------------------------------------------- L 2 error (variable u) : 0.00017053 L inf error (variable u) : 2.93867e-05 ======================================================================= EquationType: Helmholtz Session Name: Helmholtz2D Spatial Dim.: 2 Max SEM Exp. Order: 7 Expansion Dim.: 2 Advection Type: Projection Type: Discontinuous Galerkin Lambda: 0 Forcing func [0]: -(Lambda + 2*alpha*alpha)*sin(alpha*x)*sin(alpha*y) ======================================================================= Writing: "Helmholtz2D.fld" (0.0316639s, XML) ------------------------------------------- Total Computation Time = 5s ------------------------------------------- L 2 error (variable u) : 3.94935 L inf error (variable u) : 1.00002 As we can see L2 and Linf of DG solution are too high. Plotting the results I figured out that the error is on the boundaries. I have got the same results for both nektar-master version and nektar-4.4.1. Am I doing something incorrect? One more short question, there is a factor "eFactorTau" in discontinuous Helmholtz solver, which is not present in continuous one. What is this factor responsible for? I cannot run the solver not specifying it, but I get the same results for factors[StdRegions::eFactorTau] = 1.0 and factors[StdRegions::eFactorTau] = 1000. Thank you very much and Happy New Year, Aidyn