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