  A time-reversal invariant topological insulator can be generally defined by
the effective topological field theory with a quantized \theta coefficient,
which can only take values of 0 or \pi. This theory is generally valid for an
arbitrarily interacting system and the quantization of the \theta invariant can
be directly measured experimentally. Reduced to the case of a non-interacting
system, the \theta invariant can be expressed as an integral over the entire
three dimensional Brillouin zone. Alternatively, non-interacting insulators can
be classified by topological invariants defined over discrete time-reversal
invariant momenta. In this paper, we show the complete equivalence between the
integral and the discrete invariants of the topological insulator.
