  The pair interaction between magnetic flux lines in a semi-infinite slab of
an anisotropic type-II superconductor in an external field is derived in the
London limit. The case where the applied field is normal to the
superconductor/vacuum interface is considered. The presence of stray fields
near the surface leads to an additional contribution to the repulsive
interaction between flux lines that vanishes exponentially with the distance
from the interface. The pair interaction is used to obtain the continuum
elastic energy of a distorted semi-infinite flux-line array. The presence of
the superconductor/vacuum interface yields surface contributions to the
compressional and tilt elastic constants.
