In this document, we demonstrate how to solve a 1D eigenproblem (eigenvalues of the shifted 1D Laplace operator in a bounded domain) by creating custom elements. The tutorial is related to the harmonic eigenproblem , which you should read first. The fundamental difference in this case, is that we treat the eigenproblem as two coupled firstorder equations which ensures that many of the eigenvalues are complex.
in the domain , with homogeneous Dirichlet boundary conditions

We shall treat as an eigenvalue (unknown), but the other parameter is a shift, which is used to modify the spectrum of the operator. The two firstorder equations may be combined into a single secondorder equation
which is exactly the same equation as the harmonic eigenproblem , but with a redefinition of the eigenvalues. Hence, the exact solutions are given by the countably infinite set:
which yields
If the shift , then the eigenvalues are all imaginary and consist of the complex conjugate pairs given by .
If , then the first complex conjugate pair of imaginary eigenvalues becomes two distinct real eigenvalues because the term under the square root in (3) becomes positive. As increases further subsequent complex eigenvalues merge to be real. Thus, we can control the spectrum by modifying the parameter .
We now provide a detailed discussion of the driver code complex_harmonic.cc which solves the problem for the first four eigenvalues, when
If we wish to solve the problem (1) using finite elements, we simply multiply each equation by a test function, but do not integrate by parts. We shall use the same test functions for each variable and thus our governing equations become
If we expand and in terms of known basis functions and use the same basis functions as our test functions (Galerkin method), then the weak form becomes
Thus, in order to form the discrete eigenproblem , we choose to order the unknowns in the form , leading to the block Jacobian and mass matrices
where
The implementation closely follows that in the harmonic eigenproblem , and so we shall concentrate on the differences from that problem.
For generality, we implement the mathematics to assemble contributions to the Jacobian and mass matrices defined above in the class ComplexHarmonicEquations
that inherits from FiniteElement
.
The unknowns that represent the discretised eigenfunction are assumed to be stored at the nodes, but there are now two unknowns: is assumed to be the first and is the second.
As before, the key function is fill_in_contribution_to_jacobian_and_mass_matrix
which implements the calculation of the equations.
Note that the shift is implemented in a global namespace as EigenproblemShift::Mu
.
The shape functions are specified in the QComplexHarmonicElement
class that inherits from our standard onedimensional Lagrange elements QElement<1,NNODE_1D>
as well as HarmonicEquations
. The number of unknowns (two) is specified and the output functions and shape functions are overloaded as required: the output functions are specified in the ComplexHarmonicEquations
class, whereas the shape functions are provided by the QElement<1,NNODE_1D>
class.
The driver code is identical to that of the harmonic eigenproblem .
The ComplexHarmonicProblem
is derived from oomphlib's
generic Problem
class and the specific element type and eigensolver are specified as template parameters to make it easy for the "user" to change either of these from the driver code. Once again, it is very similar to the HarmonicProblem
class in the harmonic eigenproblem . The only member function with any differences is the solve
(...) function, which requests 7 rather than 4 eigenvalues to be computed and outputs the eigenfunction associated with eigenvalue of smallest magnitude (which is real).
A pdf version of this document is available.