Let is even, is odd, is even, is odd, f \left(\right. x \left.\right) = \left{\right. x - 1 , x \textrm{ }\text{is even},\textrm{ } \\ 2 x , x \textrm{ }\text{is odd},\textrm{ } x \in \mathbf{N}.
If for some a \in \mathbf{N} , f \left(\right. f \left(\right. f \left(\right. a \left.\right) \left.\right) \left.\right) = 21, then \underset{x \rightarrow a^{-}}{lim} \left{\right. \frac{\left|\right. x \left|\right.^{3}}{a} - \left[\right. \frac{x}{a} \left]\right. \left.\right}, where \left[\right. t \left]\right. denotes the greatest integer less than or equal to t, is equal to :