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ω=(ω_0, ω_1, ..., ω_n)  \end{equation}  In this case, $ω_b ∈ {\Bbb Z}^2$, $ω_0=x$, $|ω_b− ω_{b−1}| $|ω_b−ω_{b−1}|  = 1$, $b = 1, 2, . . . , n$, $ω_a \neq ω_b$ for $a \neq b$, and $0 ≤ a < b ≤ n$. Let $|ω| = N$ denote the length of $ω$.