  We demonstrate that an arbitrary system of screw dislocations in a smectic-A
liquid crystal may be consistently treated within harmonic elasticity theory,
provided that the angles between dislocations are sufficiently small. Using
this theory, we calculate the ground state configuration of the TGB-A phase. We
obtain an estimate of the twist-grain-boundary spacing and screw dislocation
spacing in a boundary in terms of the macroscopic parameters, in reasonable
agreement with experimental results.
