Counting 4口·1①，43，t夏，里)Q0 Hengfeng Wei (hfweixinju.edu.cn) 2-3 Counting March12,20205/34
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Counting Sums Σ 4口·￥①，43，t夏，3)Q0 Hengfeng Wei (hfweixinju.edu.cn) 2-3 Counting March12,20205/34
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Counting Sums ∑ Binomials 4口·1①，43，t夏，里)Q0 Hengfeng Wei (hfweixinju.edu.cn) 2-3 Counting farch12.20205/34
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PRELIMINARY 4口·￥①，43，t夏，3)Q0 Hengfeng Wei (hfweixinju.edu.cn) 2-3 Counting March12,20206/34
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Falling and Rising Factorials m2=mn=m(m-1)(m-2(m-n+1)=(m-m m! 4口·1①，43，t夏，里)Q0 Hengfeng Wei (hfweixinju.edu.cn) 2-3 Counting farch12.20207/34
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Falling and Rising Factorials mn = (m)n = m(m − 1)(m − 2)· · ·(m − n + 1) = m! (m − n)! mn¯ = m(n) = m(m + 1)(m + 2)· · ·(m + n − 1) n! = n n = 1 n¯ m n ! = mn n! Hengfeng Wei (hfwei@nju.edu.cn) 2-3 Counting March 12, 2020 7 / 34