  Neutrino oscillations are one of the most studied and successful phenomena
since the establishment of the solar neutrino problem in late 1960's. In this
work we discuss the exact expressions for the probability P_{\alpha\beta} in a
constant density medium, in terms of the standard vacuum parameters and the
medium density. Besides of being compact, these expressions are independent of
any particular parametrization, which could be helpful in the application of
unitary tests of the mixing matrix. In addition, we introduce a new
approximation on P_{\alpha\beta} and compare it with the most commonly used,
discussing their main differences.
