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任何《数学分析》教科书都支持\(\displaystyle\lim_{n \to \infty}n\in\mathbb{N}\),及与之逻辑等价的任何命题。现在我们根据Weierstrass 极限定义直接证明\(\displaystyle\lim_{n \to \infty}n\)、\(\displaystyle\lim_{n \to \infty}2n\)、\(\displaystyle\lim_{n \to \infty}2^n\)、\(\displaystyle\lim_{n \to \infty}10^n\)、……\(\in\mathbb{N}\)!
〖证明:〗根据Weierstrass极限定义:\(\displaystyle\lim_{n \to \infty}x_n=a\)\(对\forall \varepsilon>0\iff \exists\)正整数\(N_\varepsilon\)\((=[\tfrac{1}{\varepsilon}]+1)\),当\(n>N_{\varepsilon}\),有\(|x_n-a|<{\varepsilon}\),令\(\varepsilon=(\displaystyle\lim_{n \to \infty}n)^{-1}\),则\(N_\varepsilon\)\(=\displaystyle\lim_{n \to \infty}n\)\(\in\mathbb{N}\)
同理:
令\(\varepsilon=(\displaystyle\lim_{n \to \infty}2n)^{-1}\),则\(N_\varepsilon\)\(=\displaystyle\lim_{n \to \infty}2n\)\(\in\mathbb{N}\);
令\(\varepsilon=(\displaystyle\lim_{n \to \infty}2^n)^{-1}\),则\(N_\varepsilon\)\(=\displaystyle\lim_{n \to \infty}2^n\)\(\in\mathbb{N}\);
令\(\varepsilon=(\displaystyle\lim_{n \to \infty}10^n)^{-1}\),则\(N_\varepsilon\)\(=\displaystyle\lim_{n \to \infty}10^n\)\(\in\mathbb{N}\);
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