In a simple file, the initial density is fixed, namely, x 0 , j = j Δ Δ , where Δ Δ is a parameter that represents a microscopic length. The PDFs' coordinates must obey the order: x − − M ≤ ≤ x − − M + 1 ≤ ≤ ⋯ ⋯ ≤ ≤ x M .