Let [·] denote the greatest integer function and f(x)= lim_n → ∞ 1/(n^3) Σ_k =1^n [(k^2)/3^x]. Then 12 Σ_j =1^∞ f( j ) is equal to ____.

Let $\displaystyle [\cdot]$ denote the greatest integer function and $\displaystyle f(x)=\lim _{\mathrm{n} \rightarrow \infty} \frac{1}{\mathrm{n}^3} \sum_{\mathrm{k}=1}^{\mathrm{n}}\left[\frac{\mathrm{k}^2}{3^x}\right]$. Then $\displaystyle 12 \sum_{\mathrm{j}=1}^{\infty} f(\mathrm{j})$ is equal to $\displaystyle \_\_\_\_$.