  The eigenvalue problem of the Hamiltonian of an electron confined to a plane
and subjected to a perpendicular time-independent magnetic field which is the
sum of a homogeneous field and an additional field contributed by a singular
flux tube, i.e. of zero width, is investigated. Since both a direct approach
based on distribution-valued operators and a limit process starting from a
non-singular flux tube, i.e. of finite size, fail, an alternative method is
applied leading to consistent results. An essential feature is quantum
mechanical supersymmetry at g=2 which imposes, by proper representation, the
correct choice of "boundary conditions". The corresponding representation of
the Hilbert space in coordinate space differs from the usual space of
square-integrable 2-spinors, entailing other unusual properties. The analysis
is extended to $g\ne 2$ so that supersymmetry is explicitly broken. Finally,
the singular Aharonov-Bohm system with the same amount of singular flux is
analysed by making use of the fact that the Hilbert space must be the same.
