定义
- 差分算子 Δ \Delta Δ: Δ f ( x ) = f ( x + 1 ) − f ( x ) \Delta f(x)=f(x+1)-f(x) Δf(x)=f(x+1)−f(x)
- 平移算子 E E E: E f ( x ) = f ( x + 1 ) E f(x)=f(x+1) Ef(x)=f(x+1)
- 下降幂: n > 0 , { x n ‾ = x ( x − 1 ) ( x − 2 ) . . . ( x − n + 1 ) x − n ‾ = 1 ( x + 1 ) ( x + 2 ) ( x + 3 ) . . . ( x + n ) n>0,\begin{cases}x^{\underline{n}}=x(x-1)(x-2)...(x-n+1)\\x^{\underline{-n}}=\frac{1}{(x+1)(x+2)(x+3)...(x+n)}\end{cases} n>0,{ xn=x(x−1)(x−2)...(x−n+1)x−n=(x+1)(x+2)(x+3)...(x+n)1
- 上升幂: n > 0 , { x n ‾ = x ( x + 1 ) ( x + 2 ) . . . ( x + n − 1 ) x − n ‾ = 1 ( x − 1 ) ( x − 2 ) ( x − 3 ) . . . ( x − n ) n>0,\begin{cases}x^{\overline{n}}=x(x+1)(x+2)...(x+n-1)\\x^{\overline{-n}}=\frac{1}{(x-1)(x-2)(x-3)...(x-n)}\end{cases} n>0,{ xn=x(x+1)(x+2)...(x+n−1)x−n=(x−1)(x−2)(x−3)...(x−n)1
- 阶乘幂(下降幂和上升幂的统称)的简单性质:
- x a + b ‾ = x a ‾ ( x − a ) b ‾ x^{\underline{a+b}}=x^{\underline{a}}(x-a)^{\underline{b}} xa+b=xa(x−a)b
- x a + b ‾ = x a ‾ ( x + a ) b ‾ x^{\overline{a+b}}=x^{\overline{a}}(x+a)^{\overline{b}} xa+b=xa(x+a)b
- x n ‾ = ( − 1 ) n ( − x ) n ‾ x^{\underline{n}}=(-1)^n(-x)^{\overline{n}} xn=(−1)n(−x)n
- x n ‾ = ( − 1 ) n ( − x ) n ‾ x^{\overline{n}}=(-1)^n(-x)^{\underline{n}} xn=(−1)n(−x)n
- x k ‾ ( x − 1 2 ) k ‾ = ( 2 x ) 2 k ‾ 2 2 k x^{\underline{k}}(x-\frac{1}{2})^{\underline{k}}=\frac{(2x)^{\underline{2k}}}{2^{2k}} xk(x−21)k=22k(2x)2k
证明: x k ‾ ( x − 1 2 ) k ‾ = x ( x − 1 2 ) ( x − 1 ) ( x − 1 − 1 2 ) . . . ( x − k + 1 ) ( x − k + 1 − 1 2 ) = 2 − 2 k × 2 x ( 2 x − 1 ) ( 2 x − 2 ) ( 2 x − 3 ) . . . ( 2 x − 2 k + 2 ) ( 2 x − 2 k + 1 ) = ( 2 x ) 2 k ‾ 2 2 k x^{\underline{k}}(x-\frac{1}{2})^{\underline{k}}\\=x(x-\frac{1}{2})(x-1)(x-1-\frac{1}{2})...(x-k+1)(x-k+1-\frac{1}{2})\\=2^{-2k}\times 2x(2x-1)(2x-2)(2x-3)...(2x-2k+2)(2x-2k+1)\\=\frac{(2x)^{\underline{2k}}}{2^{2k}} xk(x−21)k=x(x−21)(x−1)(x−1−21)...(x−k+1)(x−k+1−21)=2−2k×2x(2x−1)(2x−2)(2x−3)...(2x−2k+2)(2x−2k+1)=22k(2x)2k - 当 n n n 是自然数时,有
( x + y ) n ‾ = ∑ i = 0 n ( n i ) x i ‾ y n − i ‾ (x+y)^{\underline{n}}=\sum\limits_{i=0}^n\dbinom{n}{i}x^{\underline i}y^{\underline{n-i}} (x+y)n=i=0∑n(in)xiyn−i
( x + y ) n ‾ = ∑ i = 0 n ( n i ) x i ‾ y n − i ‾ (x+y)^{\overline{n}}=\sum\limits_{i=0}^n\dbinom{n}{i}x^{\overline i}y^{\overline{n-i}} (x+y)n=i=0∑n(in)xiyn−i
有限微积分
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Δ ( x m ‾ ) = m x m − 1 ‾ \Delta(x^{\underline{m}})=mx^{\underline{m-1}} Δ(xm)=mxm−1 (类比求导)
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∑ i = 0 n − 1 i k ‾ = n k + 1 ‾ k + 1 \sum_{i=0}^{n-1}i^{\underline{k}}=\frac{n^{\underline{k+1}}}{k+1} ∑i=0n−1ik=k+1nk+1 (类比经典积分 ∫ x k = x k + 1 k + 1 \int x^k=\frac{x^{k+1}}{k+1} ∫xk=k+1xk+1)
特例:经典积分中 k = − 1 k=-1 k=−1时有特例 ∫ 1 x = ln x \int \frac{1}{x}=\ln x ∫x1=lnx,这里也有 ∑ i = 0 n − 1 i − 1 ‾ = ∑ i = 0 n − 1 1 i + 1 = ∑ i = 1 n 1 i \sum_{i=0}^{n-1}i^{\underline{-1}}=\sum_{i=0}^{n-1}\frac{1}{i+1}=\sum_{i=1}^{n}\frac{1}{i} ∑i=0n−1i−1=∑i=0n−1i+11=∑i=1ni1
(调和级数: ∑ i = 0 ∞ 1 i \sum_{i=0}^{\infty}\frac{1}{i} ∑i=0∞i1) -
Δ ( 2 x ) = 2 x \Delta(2^x)=2^x Δ(2x)=2x (类比 ( e x ) ′ = e x (e^x)\prime=e^x (ex)′=ex)
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乘法法则: Δ ( u v ) = u ⋅ Δ v + E v ⋅ Δ u \Delta(uv)=u\cdot\Delta v+Ev\cdot\Delta u Δ(uv)=u⋅Δv+Ev⋅Δu
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分部积分法则: ∑ u ⋅ Δ v = u v − ∑ E v ⋅ Δ u \sum u\cdot \Delta v=uv-\sum Ev\cdot \Delta u ∑u⋅Δv=uv−∑Ev⋅Δu