Let \( f(x) = \lim_{n \to \infty} \left( \frac{1}{n^3} \sum_{k=1}^{n} \left\lfloor \frac{k^2}{3^x} \right\rfloor \right) \), where \( \left\lfloor . \right\rfloor \) denotes the greatest integer function, then \( 12 \sum_{j=1}^{\infty} f(j) \) is equal to: