(積分)均值定理(Mean-value theorem)

  • Theorem: 函數α:[a,b]R \alpha:[a,b] \rightarrow \mathbb{R}為遞增函數,且函數fR(α)[a,b]f \in \mathbf{R}(\alpha)[a,b]
    • M=sup{f(x)x[a,b]} M = \sup \lbrace f(x) \vert x \in [a,b] \rbrace .
    • m=inf{f(x)x[a,b]} m = \inf \lbrace f(x) \vert x \in [a,b] \rbrace .
    • then cR, mcM \exists c \in \mathbb{R},\ m \leq c \leq M \ni
    • abf(x)dα(x)=cabdα(x)=c[α(b)α(a)]. \begin{array}{rcl} \int_a^b f(x) d \alpha(x) & = & c \int_a^b d \alpha(x) \\ & = & c[\alpha(b) - \alpha(a)]. \end{array}
    • 若函數ff在區間[a,b][a,b]連續時,c=f(x0)c = f(x_0) for some x0[a,b]x_0 \in [a,b].
  • Integral MVT
    積分均值定理
    • Proof:
    • α(a)=α(b)\alpha(a) = \alpha(b)時,因為積分值為0,所以必定成立。
    • 考慮α(a)<α(b)\alpha(a) < \alpha(b),可得不等式:
    • m[α(b)α(a)]L(P,f,α)U(P,f,α)M[α(b)α(a)] m[\alpha(b) - \alpha(a)] \leq L(P,f,\alpha) \leq U(P,f,\alpha) \leq M[\alpha(b) - \alpha(a)].
    • 因此 m[α(b)α(a)]abfdαM[α(b)α(a)]m[\alpha(b) - \alpha(a)] \leq \int_a^b f d\alpha \leq M[\alpha(b) - \alpha(a)].
    • mabfdαα(b)α(a)M, abdα(x)=α(b)α(a)\therefore m \leq \frac{\int_a^b f d\alpha}{\alpha(b) - \alpha(a)} \leq M, \ \int_a^b d\alpha(x) = \alpha(b) - \alpha(a).
    • mabfdαabdα(x)M\therefore m \leq \frac{\int_a^b f d\alpha}{\int_a^b d \alpha(x)} \leq M.(QED)
  • Theorem: 函數α\alpha為連續函數且函數ff在區間[a,b][a,b]為遞增函數。x0[a,b]abf(x)dα(x)=f(a)ax0dα(x)+f(b)x0bdα(x)\exists x_0 \in [a,b] \ni \int_a^b f(x)d \alpha(x) = f(a) \int_a^{x_0} d \alpha(x) + f(b) \int_{x_0}^b d \alpha(x) .
    • Proof:
    • 由分部積分 abf(x)dα(x)=f(b)α(b)f(a)α(a)abα(x)df(x). \int_a^b f(x) d \alpha(x) = f(b)\alpha(b) - f(a)\alpha(a) - \int_a^b \alpha(x)df(x).
    • 由均值定理得 abf(x)dα(x)=f(a)[α(x0)α(a)]+f(b)[α(b)α(x0)]. x0[a,b]. \int_a^b f(x) d \alpha(x) = f(a)[\alpha(x_0) - \alpha(a)] + f(b)[\alpha(b) - \alpha(x_0)].\ x_0 \in [a,b]. (QED).

積分為區間長度的函數

    1. Theorem: 函數α:[a,b]R \alpha: [a,b] \rightarrow \mathbb{R}為有界變分,且函數fRS(α)[a,b]f \in \mathbf{RS}(\alpha)[a,b]。令函數F(x)=axfdα, x[a,b]F(x) = \int_a^x f d \alpha, \ x \in [a,b].可得
    2. FF在區間[a,b][a,b]為有界變分。
    3. 在函數α\alpha上連續的點,在函數FF上也連續。
    4. 若函數α:[a,b]R\alpha:[a,b] \rightarrow \mathbb{R}為遞增函數,當x(a,b)\forall x \in (a,b)α(x)\alpha^{'}(x)存在且ff為連續函數時,微分函數F(x)F^{'}(x)存在,且F(x)=f(x)α(x)F^{'}(x) = f(x) \alpha^{'}(x) .
    • Proof(1)(2):
    • 假設α\alpha[a,b][a,b]為遞增函數,所以α\alpha在此區間為有界變分。
    • xyx \neq y,根據MVT可得F(y)F(x)=xyfdα=c[α(y)α(x)], mcMF(y) - F(x) = \int_x^y f d \alpha = c[\alpha(y) - \alpha(x)],\ m \leq c \leq M [1].
    • m=inf{f(x)x[a,b]}, m=sup{f(x)x[a,b]}m = \inf \{ f(x) | x \in [a,b] \},\ m = \sup \{f(x) | x \in [a,b] \}.
    • 由[1]可得FF為有界變分且連續 (QED)。
    • Proof (3):
    • 將[1]兩側除以yxy-x 且觀察cf(x)c \rightarrow f(x) as yxy \rightarrow x (QED)。

微積分第二基本定理(Second fundmanetal theorem of calculus)

  • 令函數fRS[a,b]f\in \mathbf{RS}[a,b];函數g:[a,b]Rg:[a,b] \rightarrow \mathbb{R},且其微分g(x)=f(x),x(a,b)g^{'}(x) = f(x), \forall x \in (a,b)存在。端點值g(a+), g(b)g(a+),\ g(b-) 存在且滿足g(a)g(a+)=g(b)g(b)g(a) - g(a+) = g(b) - g(b-),則可得abf(x)dx=abg(x)dx=g(b)g(a) \int_a^b f(x) dx = \int_a^b g^{'}(x) dx = g(b) - g(a) .

    • Proof:
    • 對於[a,b][a,b]的任意分割,可得下式
    • g(b)g(a)=k=1n[g(xk)g(xk1)]=k=1ng(tk)Δxk(byMVT)=k=1nf(tk)Δxk.tk(xk1,xk).\begin{array}{rcl} g(b) - g(a) & = & \sum_{k=1}^n [g(x_k) - g(x_{k-1})] \\ & = & \sum_{k=1}^n g^{'}(t_k) \Delta x_k \\ (by MVT) & = & \sum_{k=1}^n f(t_k) \Delta x_k. \\ & & t_k \in (x_{k-1}, x_k). \end{array}

    • Given ϵ>0\epsilon > 0 , 分割切的更細後可得以下性質

    • g(b)g(a)abf(x)dx=k=1nf(tk)Δxkabf(x)dx<ϵ \left| g(b) - g(a) -\int_a^b f(x) dx \right| = \left| \sum_{k=1}^n f(t_k) \Delta x_k - \int_a^b f(x) dx \right| < \epsilon (QED)
  • 函數fRS(α)[a,b]f \in \mathbf{RS}(\alpha)[a,b],函數αC[a,b]\alpha \in \mathbf{C}[a,b]且其微分αR[a,b]\alpha^{'} \in \mathbf{R}[a,b],則下式存在 abf(x)dα(x)=abf(x)α(x)dx\int_a^b f(x) d\alpha(x) = \int_a^b f(x) \alpha^{'}(x) dx .

    • Proof:
    • 由第二基本定理知x[a,b], α(x)α(a)=axα(t)dt\forall x \in [a,b], \ \alpha(x) - \alpha(a) = \int_a^x \alpha^{'}(t) dt
    • 由變數變換可得到結果(QED).

Riemann積分變數變換

  • g(c)g(d)f(x)dx=cdf[g(t)]g(t)dt \int_{g(c)}^{g(d)} f(x) dx = \int_c^d f[g(t)]g^{'}(t) dt
  • 函數gC[a,b]g \in \mathbf{C}[a,b],且微分gC[a,b]g^{'} \in \mathbf{C}[a,b]存在。函數fC[c,d]f \in \mathbf{C}[c,d]且定義F(x)=g(c)xf(t)dt, xg([c,d])F(x) = \int_{g(c)}^x f(t) dt, \ x \in g([c,d])。則 x[c,d]\forall x \in [c,d],積分cxf[g(t)]g(t)dt\int_c^x f[g(t)]g^{'}(t) dt存在且g(c)g(d)f(x)dx=cdf[g(t)]g(t)dt\int_{g(c)}^{g(d)} f(x) dx = \int_c^d f[g(t)]g^{'}(t) dt .

Riemann第二積分均值定理

    1. 函數f, gC[a,b]f,\ g \in \mathbf{C}[a,b],且ff在此區間為遞增函數。令A,BRA, B \in \mathbb{R}滿足Af(a+) and Bf(b)A \leq f(a+) \text{ and } B \geq f(b-),則 x0[a,b]\exists x_0 \in [a,b] \ni
    2. abf(x)g(x)dx=Aax0g(x)dx+Bx0bg(x)dx.\int_a^b f(x) g(x) dx = A \int_a^{x_0} g(x) dx + B \int_{x_0}^b g(x) dx.
    3. (Bonnet's theorem) if f(x)0, x[a,b]f(x) \geq 0,\ \forall x \in [a,b] then abf(x)g(x)dx=Bx0bg(x)dx, x0[a,b].\int_a^b f(x) g(x) dx = B \int_{x_0}^b g(x) dx , \ x_0 \in [a,b].
    • Proof (1):
    • α(x)=axg(t)dt\alpha(x) = \int_a^x g(t) dt,則α=g\alpha^{'} = g
    • 根據MVT可得abf(x)g(x)dx=f(a)x0ag(x)dx+f(b)x0bg(x)dx\int_a^b f(x) g(x) dx = f(a) \int^a_{x_0} g(x) dx + f(b) \int_{x_0}^b g(x) dx .
    • A=f(a)A = f(a)B=f(b)B=f(b),則(1) 成立。(QED).
    • Proof (2), 因為改變函數單點的值不會影響積分的結果,令A=0A=0得證(QED)。

Riemann-Stieltjes積分為函數

  • 在二重積分時,函數f(x,y)f(x,y)給定不同的y=y0y=y_0所對應的到的積分區域均不相同,所以此時積分為yy的函數;同理給定x=x0x=x_0也會得到不同的積分區域。

    • 函數ff在長方形區間Q={(x,y)axb, cyd}Q=\{ (x,y) | a \leq x \leq b,\ c \leq y \leq d \} 連續。
    • 假設函數α\alpha在區間[a,b][a,b]為有界變分,且函數F(y)=abf(x,y)dα(x), y[c,d].F(y)=\int_a^b f(x,y)d \alpha(x),\ y \in [c,d].,則FC[c,d]F \in \mathbf{C}[c,d]
    • 可解釋為y0[c,d]y_0 \in [c,d], 則 limyy0abf(x,y)dα(x)=ablimyy0f(x,y)dα(x)=abf(x,y0)dα(x)\lim_{y \rightarrow y_0} \int_a^b f(x,y) d \alpha(x) = \int_a^b \lim_{y \rightarrow y_0} f(x,y) d \alpha(x) = \int_a^b f(x, y_0) d\alpha(x)
    • .
    • Proof:
    • α:[a,b]R\alpha:[a,b] \rightarrow \mathbb{R}為遞增函數。
    • 因為QQ為compact set,所以ff為uniformly continuous on QQ
    • ϵ>0δ>0z1=(x1,y1), z2=(x2,y2), z1z2<δ, f(x1,y1)f(x2,y2)<ϵ\therefore \forall \epsilon > 0 \exists \delta > 0 \ni z_1=(x_1,y_1),\ z_2=(x_2,y_2),\ |z_1 - z_2| < \delta, \ |f(x_1, y_1) - f(x_2, y_2) | < \epsilon .
    • If y1y2<δ |y_1 - y_2 | < \delta,可得F(y1)F(y2)abf(x1,y1)f(x2,y2)dα(x)ϵ[α(b)α(a)]| F(y_1) - F(y_2)| \leq \int_a^b |f(x_1, y_1) - f(x_2, y_2) |d \alpha(x) \leq \epsilon [\alpha(b) - \alpha(a)]
    • 所以FC[a,b]F \in \mathbf{C}[a,b](QED).
    • 函數fC[a,b]×[c,d]f \in \mathbf{C}[a,b] \times [c,d]
    • 函數g:[a,b]Rg: [a,b] \rightarrow \mathbb{R}
    • 定義函數F(y)=abg(x)f(x,y)dxC[c,d]F(y) = \int_a^b g(x) f(x,y) dx \in \mathbf{C}[c,d],即y0[c,d], limyy0abg(x)f(x,y)dx=abg(x)f(x,y0)dx.\forall y_0 \in [c,d], \ \lim_{y \rightarrow y_0} \int_a^b g(x) f(x,y)dx = \int_a^b g(x) f(x,y_0)dx.

    • Proof:
    • Let G(x)=axg(t)dtG(x) = \int_a^x g(t) dt,則F(y)=abf(x,y)dG(x)F(y) = \int_a^b f(x,y) d G(x)。(QED)

在積分內的微分

    • Q={(x,y)axb, cyd} Q = \{(x,y)| a \leq x \leq b, \ c \leq y \leq d \}
    • 函數α:QR\alpha:Q \rightarrow \mathbb{R}[a,b][a,b]為有界變分,但是在[c,d][c,d]為定值。
    • 假設積分F(y)=abf(x,y)dα(x)<F(y) = \int_a^b f(x,y) d\alpha(x) < \infty
    • 若偏微分DyfD_yfQQ連續,則微分值存在且y[c,d], F(y)=abDyf(x,y)dα(x)\forall y \in [c,d], \ F^{'}(y) = \int_a^b D_y f(x,y) d \alpha(x)
    • Proof:
    • If y0(c,d), yy0y_0 \in (c,d), \ y \neq y_0, then
    • F(y)F(y0)yy0=abf(x,y)f(x,y0)yy0dα(x)=abDyf(x,y1)dα(x), y1[y,y0]\frac{F(y) - F(y_0)}{y-y_0} = \int_a^b \frac{f(x,y) - f(x, y_0)}{y - y_0} d \alpha(x) = \int_a^b D_y f(x,y_1) d \alpha(x),\ y_1 \in [y, y_0].
    • 因為DyfD_y fQQ連續,因此得證(QED).

變更積分順序

    • Q={(x,y)axb,cyd} Q = \{ (x,y) | a \leq x \leq b, c \leq y \leq d\}
    • 函數α:[a,b]R\alpha:[a,b] \rightarrow \mathbb{R}為有界變分,且函數β:[c,d]R\beta: [c,d] \rightarrow \mathbb{R} 也是有界變分,函數fC(Q)f \in \mathbf{C}(Q)
    • F(y)=abf(x,y)dα(x), G(x)=cdf(x,y)dβ(y)F(y) = \int_a^b f(x,y) d \alpha(x), \ G(x) = \int_c^d f(x,y) d \beta(y)
    • FRS(β)[c,d], GRS(α)[a,b]F \in \mathbf{RS}(\beta)[c,d], \ G \in \mathbf{RS}(\alpha)[a,b]cdF(y)dβ(y)=abG(x)dα(x)\int_c^d F(y) d \beta(y) = \int_a^b G(x) d \alpha(x)
    • ab[cdf(x,y)dβ(y)]dα(x)=cd[abf(x,y)dα(x)]dβ(y)\int_a^b \left[ \int_c^d f(x,y) d \beta(y) \right] d \alpha(x) = \int_c^d \left[ \int_a^b f(x,y) d \alpha(x) \right] d \beta(y)

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