Low-Autocorrelation Binary Sequence Problem

$S = \{s_{1},s_{2}, ..., s_{L} \};~~~s_{i} \in \{+1, -1\}$

$C_{k}(S) = \sum_{i=1}^{L-k}s_{i} \cdot s_{i+k}$

$E(S)=\sum_{k=1}^{L-1}{C_{k}^{2}(S)}$

$F(S) = \frac{L^{2}}{2 \cdot E(S)}$