  It is shown that the linear approximation of the central caustic for the
planetary ($q\ll1$) microlensing is valid if $|1-s|\gg q^{1/3}$ (where $q$ is
the mass ratio and $s$ is the projected separation in the unit of the Einstein
ring radius of the primary). The condition is also consistent with the
requirement that the binary separation is far from those in the resonant binary
regime resulting in a single six-cusp caustic. Given that the linear
approximation of the caustic is invariant under $s\leftrightarrow s^{-1}$, the
close/wide binary degeneracy observed under the same condition may be
understood via the linear approximation of the central caustic. Finally it is
argued that the local degeneracies of lensing features associated with caustic
crossings can still persist in the planetary events even when $|1-s|\sim
q^{1/3}$ although the overall caustic shape may not be degenerate at all.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics;Astrophysics of Galaxies