formula

$X_N^{(2)} &=& \frac{X_0}{N}R_1 R_2 \cdots R_N \frac{X_0}{N} R_2 R_3 \cdots R_N \cdots \frac{X_0}{N}R_N \\ &=& \frac{X_0}{N} (R_1 \cdots R_N R_2 \cdots R_N \cdots R_N) \\ &=& \frac{X_0}{N} \sum_{i=1}^N \prod_{j=i}^N R_j \\ &\equiv& \frac{X_0}{N} \sum_{i=1}^N {\cal R}_i$