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

On the condition for the central caustic degeneracy of the planetary microlensing