2. 数列极限
2.2 数列极限
2.2.5 数列极限的性质
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保序性:【定理2.2.3】 { x n } , { y n } , lim n → ∞ x n = a , lim n → ∞ y n = b , a < b \{x_{n}\},\{y_{n}\},\lim\limits_{n\to\infty}x_{n}=a,\lim\limits_{n\to\infty}y_{n}=b,a<b {xn},{yn},n→∞limxn=a,n→∞limyn=b,a<b,则存在 N N N, ∀ n > N \forall n>N ∀n>N,成立 x n < y n x_{n}<y_{n} xn<yn.
【证】取 ε = b − a 2 > 0 \varepsilon=\frac{b-a}{2}>0 ε=2b−a>0(任意给定,取一个巧妙的值,实际上是逆推得到,只要保证大于0就行),
存在 N 1 , ∀ n > N 1 : ∣ x n − a ∣ < ε N_{1},\forall n>N_{1}:|x_{n}-a|<\varepsilon N1,∀n>N1:∣xn−a∣<ε,即 a − ε < x n < a + ε a-\varepsilon<x_{n}<a+\varepsilon a−ε<xn<a+ε,亦即 x n < a + ε = a + b − a 2 = a + b 2 x_{n}<a+\varepsilon=a+\frac{b-a}{2}=\frac{a+b}{2} xn<a+ε=a+2b−a=2a+b,
存在 N 2 , ∀ n < N 2 : ∣ y n − b ∣ < ε N_{2},\forall n<N_{2}:|y_{n}-b|<\varepsilon N2,∀n<N2:∣yn−b∣<ε,即 b − ε < y n < b + ε b-\varepsilon<y_{n}<b+\varepsilon b−ε<yn<b+ε,亦即 y n > b − ε = b − b − a 2 = a + b 2 y_{n}>b-\varepsilon=b-\frac{b-a}{2}=\frac{a+b}{2} yn>b−ε=b−2b−a=2a+b
取 N = max { N 1 , N 2 } , ∀ n > N , x n < a + b 2 < y n N=\max\{N_{1},N_{2}\},\forall n>N, x_{n}<\frac{a+b}{2}<y_{n} N=max{N1,N2},∀n>N,xn<2a+b<yn
【逆命题】 lim n → ∞ x n = a , lim n → ∞ y n = b \lim\limits_{n\to\infty}x_{n}=a,\lim\limits_{n\to\infty}y_{n}=b n→∞limxn=a,n→∞limyn=b,若 x n < y n x_{n}<y_{n} xn<yn,能否推出 a < b a<b a<b?
【举反例】 x n = 1 n , y n = 2 n x_{n}=\frac{1}{n},y_{n}=\frac{2}{n} xn=n1,yn=n2, ∀ n , x n < y n \forall n,x_{n}<y_{n} ∀n,xn<yn,但是 lim n → ∞ x n = 0 = lim n → ∞ y n = 0 \lim\limits_{n\to\infty}x_{n}=0=\lim\limits_{n\to\infty}y_{n}=0 n→∞limxn=0=n→∞limyn=0 -
【命题】若 lim n → ∞ x n = a , lim n → ∞ y n = b \lim\limits_{n\to\infty}x_{n}=a,\lim\limits_{n\to\infty}y_{n}=b n→∞limxn=a,n→∞limyn=b,若 ∃ N , ∀ n > N , x n ≤ y n \exists N,\forall n>N,x_{n}\le y_{n} ∃N,∀n>N,xn≤yn,则 a ≤ b a\le b a≤b.
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【推论1】若 lim n → ∞ y n = b > 0 \lim\limits_{n\to\infty}y_{n}=b>0 n→∞limyn=b>0,则 ∃ N , ∀ n > N : y n > b 2 > 0 \exists N,\forall n>N:y_{n}>\frac{b}{2}>0 ∃N,∀n>N:yn>2b>0( y n y_{n} yn从某一项开始不仅仅离开0,距离0有一段距离,而不是像 1 n \frac{1}{n} n1那样,无限地趋近于0,这个定理说明的是它不能很靠近0)
【注】证明将 b 2 \frac{b}{2} 2b视为一个常数数列,用保序性即可。 -
【推论2】 lim n → ∞ y n = b < 0 \lim\limits_{n\to\infty}y_{n}=b<0 n→∞limyn=b<0,则 ∃ N , ∀ n > N : y n < b 2 < 0 \exists N,\forall n>N:y_{n}<\frac{b}{2}<0 ∃N,∀n>N:yn<2b<0( y n < 0 y_{n}<0 yn<0但是和0保持一定的距离)
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【推论1,2合起来,数列极限的保序性定理】若 lim n → ∞ y n = b ≠ 0 \lim\limits_{n\to\infty}y_{n}=b\ne 0 n→∞limyn=b=0,则 ∃ N , ∀ n > N , ∣ y n ∣ > ∣ b ∣ 2 > 0 \exists N,\forall n>N,|y_{n}|>\frac{|b|}{2}>0 ∃N,∀n>N,∣yn∣>2∣b∣>0
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【数列极限的夹逼性定理】【定理2.2.4】, { x n } , { y n } , { z n } \{x_{n}\},\{y_{n}\},\{z_{n}\} {xn},{yn},{zn}, ∃ N , ∀ n > N : \exists N,\forall n>N: ∃N,∀n>N:成立 x n ≤ y n ≤ z n x_{n}\le y_{n}\le z_{n} xn≤yn≤zn,且 lim n → ∞ x n = lim n → ∞ z n = a \lim\limits_{n\to\infty}x_{n}=\lim\limits_{n\to\infty}z_{n}=a n→∞limxn=n→∞limzn=a,则 lim n → ∞ y n = a \lim\limits_{n\to\infty}y_{n}=a n→∞limyn=a
【证】 ∀ ε > 0 , ∃ N 1 : ∣ x n − a ∣ < ε ⇒ a − ε < x n \forall \varepsilon>0,\exists N_{1}:|x_{n}-a|<\varepsilon \Rightarrow a-\varepsilon < x_{n} ∀ε>0,∃N1:∣xn−a∣<ε⇒a−ε<xn
∃ N 2 : ∣ z n − a ∣ < ε ⇒ z n < a + ε \exists N_{2}:|z_{n}-a|<\varepsilon \Rightarrow z_{n}<a+\varepsilon ∃N2:∣zn−a∣<ε⇒zn<a+ε
取 N ′ = max { N , N 1 , N 2 } , ∀ n > N ′ : a − ε < x n ≤ y n ≤ z n < a + ε N'=\max\{N,N_{1},N_{2}\},\forall n>N':a-\varepsilon < x_{n}\le y_{n} \le z_{n} < a+\varepsilon N′=max{N,N1,N2},∀n>N′:a−ε<xn≤yn≤zn<a+ε
即 a − ε < y n < a + ε a-\varepsilon <y_{n}< a+\varepsilon a−ε<yn<a+ε
亦即 ∣ y n − a ∣ < ε |y_{n}-a|<\varepsilon ∣yn−a∣<ε
所以 lim n → ∞ y n = a \lim\limits_{n\to\infty}y_{n}=a n→∞limyn=a
【例2.2.7】求
lim
n
→
∞
(
n
+
1
−
n
)
\lim\limits_{n\to\infty}(\sqrt{n+1}-\sqrt{n})
n→∞lim(n+1−n)
【答】
0
<
n
+
1
−
n
=
(
n
+
1
−
n
)
(
n
+
1
+
n
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n
+
1
+
n
=
n
+
1
−
n
n
+
1
+
n
=
1
n
+
1
+
n
<
1
n
0<\sqrt{n+1}-\sqrt{n}=\frac{(\sqrt{n+1}-\sqrt{n})(\sqrt{n+1}+\sqrt{n})}{\sqrt{n+1}+\sqrt{n}}=\frac{n+1-n}{\sqrt{n+1}+\sqrt{n}}=\frac{1}{\sqrt{n+1}+\sqrt{n}}<\frac{1}{\sqrt{n}}
0<n+1−n=n+1+n(n+1−n)(n+1+n)=n+1+nn+1−n=n+1+n1<n1
因为
lim
n
→
∞
0
=
0
,
lim
n
→
∞
1
n
=
0
\lim\limits_{n\to\infty}0=0,\lim\limits_{n\to\infty}\frac{1}{\sqrt{n}}=0
n→∞lim0=0,n→∞limn1=0
由夹逼性定理可知
lim
n
→
∞
(
n
+
1
−
n
)
=
0
\lim\limits_{n\to\infty}(\sqrt{n+1}-\sqrt{n})=0
n→∞lim(n+1−n)=0
【例2.2.8】证明
lim
n
→
∞
(
a
1
n
+
a
2
n
+
.
.
.
+
a
p
n
)
1
n
=
max
1
≤
i
≤
p
{
a
i
}
,
a
1
,
a
2
,
.
.
.
,
a
p
>
0
\lim\limits_{n\to\infty}(a_{1}^{n}+a_{2}^{n}+...+a_{p}^{n})^{\frac{1}{n}}=\max\limits_{1\le i\le p}\{a_{i}\},a_{1},a_{2},...,a_{p}>0
n→∞lim(a1n+a2n+...+apn)n1=1≤i≤pmax{ai},a1,a2,...,ap>0
【证】不失一般性,设
a
1
=
max
1
≤
i
≤
p
{
a
i
}
a_{1}=\max\limits_{1\le i\le p}\{a_{i}\}
a1=1≤i≤pmax{ai}
a
1
=
(
a
1
n
)
1
n
≤
(
a
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+
a
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+
a
p
n
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1
n
≤
(
p
a
1
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1
n
=
a
1
p
1
n
a_{1}=(a_{1}^{n})^{\frac{1}{n}}\le (a_{1}^{n}+a_{2}^{n}+...+a_{p}^{n})^{\frac{1}{n}}\le (pa_{1}^{n})^{\frac{1}{n}}=a_{1}p^{\frac{1}{n}}
a1=(a1n)n1≤(a1n+a2n+...+apn)n1≤(pa1n)n1=a1pn1
(不等式左侧能取小于等于的情况是
a
1
=
1
a_{1}=1
a1=1,剩下的均为0)
由于
lim
n
→
∞
a
1
=
a
1
,
lim
n
→
∞
a
1
p
1
n
a
1
\lim\limits_{n\to\infty}a_{1}=a_{1},\lim\limits_{n\to\infty}a_{1}p^{\frac{1}{n}}a_{1}
n→∞lima1=a1,n→∞lima1pn1a1(此处
p
p
p是下标,下标从1开始,用之前课程证明过的结论:
a
>
1
,
lim
n
→
∞
a
n
=
1
a>1,\lim\limits_{n\to\infty}\sqrt[n]{a}=1
a>1,n→∞limna=1)
由夹逼性定理可知
lim
n
→
∞
(
a
1
n
+
a
2
n
+
.
.
.
+
a
p
n
)
1
n
=
max
1
≤
i
≤
p
{
a
i
}
\lim\limits_{n\to\infty}(a_{1}^{n}+a_{2}^{n}+...+a_{p}^{n})^{\frac{1}{n}}=\max\limits_{1\le i\le p}\{a_{i}\}
n→∞lim(a1n+a2n+...+apn)n1=1≤i≤pmax{ai}
【例】用夹逼性定理证明
lim
n
→
∞
n
n
=
1
\lim\limits_{n\to\infty}\sqrt[n]{n}=1
n→∞limnn=1
【证】
1
<
n
n
=
n
n
⋅
1
⋅
1...1
n
1<\sqrt[n]{n}=\sqrt[n]{\sqrt{n}\sqrt{n}\cdot1\cdot1...1}
1<nn=nnn⋅1⋅1...1(
n
−
2
n-2
n−2个1)
≤
2
n
+
n
−
2
n
=
1
+
2
n
−
2
n
\le\frac{2\sqrt{n}+n-2}{n}=1+\frac{2\sqrt{n}-2}{n}
≤n2n+n−2=1+n2n−2(用平均值不等式)
因为
lim
n
→
∞
(
1
+
2
n
−
2
n
)
=
1
\lim\limits_{n\to\infty}(1+\frac{2\sqrt{n}-2}{n})=1
n→∞lim(1+n2n−2)=1(后面会证明出来,视频课这里感觉讲课的逻辑有点乱)
又
lim
n
→
∞
1
=
1
\lim\limits_{n\to\infty}1=1
n→∞lim1=1
由夹逼性定理可知
lim
n
→
∞
n
n
=
1
\lim\limits_{n\to\infty}\sqrt[n]{n}=1
n→∞limnn=1
【注1】若用夹逼性定理证明
lim
n
→
∞
n
2
n
=
1
\lim\limits_{n\to\infty}\sqrt[n]{n^{2}}=1
n→∞limnn2=1,可以放缩为
1
<
n
2
n
=
n
n
n
n
⋅
1
⋅
1...1
n
1<\sqrt[n]{n^{2}}=\sqrt[n]{\sqrt{n}\sqrt{n}\sqrt{n}\sqrt{n}\cdot1\cdot1...1}
1<nn2=nnnnn⋅1⋅1...1(
n
−
4
n-4
n−4个1)
≤
4
n
+
n
−
4
n
=
1
+
4
n
−
4
n
\le\frac{4\sqrt{n}+n-4}{n}=1+\frac{4\sqrt{n}-4}{n}
≤n4n+n−4=1+n4n−4
【注2】关于
lim
n
→
∞
(
1
+
2
n
−
2
n
)
=
1
\lim\limits_{n\to\infty}(1+\frac{2\sqrt{n}-2}{n})=1
n→∞lim(1+n2n−2)=1可以有更一般性的结论(后面证明):
1)数列极限的四则运算;
2)
【注3】【平均值不等式】若有
a
1
,
a
2
,
.
.
.
,
a
n
a_{1},a_{2},...,a_{n}
a1,a2,...,an(
n
n
n个正数),将
a
1
+
a
2
+
.
.
.
+
a
n
n
\frac{a_{1}+a_{2}+...+a_{n}}{n}
na1+a2+...+an称为算术平均,把
a
1
a
2
.
.
.
a
n
n
\sqrt[n]{a_{1}a_{2}...a_{n}}
na1a2...an称为几何平均,
n
1
a
1
+
1
a
2
+
.
.
.
+
1
a
n
\frac{n}{\frac{1}{a_{1}}+\frac{1}{a_{2}}+...+\frac{1}{a_{n}}}
a11+a21+...+an1n称为调和平均,则有:
a
1
+
a
2
+
.
.
.
+
a
n
n
≥
a
1
a
2
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a
n
n
≥
n
1
a
1
+
1
a
2
+
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.
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+
1
a
n
\frac{a_{1}+a_{2}+...+a_{n}}{n}\ge \sqrt[n]{a_{1}a_{2}...a_{n}}\ge \frac{n}{\frac{1}{a_{1}}+\frac{1}{a_{2}}+...+\frac{1}{a_{n}}}
na1+a2+...+an≥na1a2...an≥a11+a21+...+an1n
等号成立的条件是
a
1
=
a
2
=
.
.
.
=
a
n
a_{1}=a_{2}=...=a_{n}
a1=a2=...=an。