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