If b is very small as compared to the value of a, so that the cube and other higher powers of ${b \over a}$ can be neglected in the identity ${1 \over {a - b}} + {1 \over {a - 2b}} + {1 \over {a - 3b}} + ..... + {1 \over {a - nb}} = \alpha n + \beta {n^2} + \gamma {n^3}$, then the value of $\gamma$ is :