走一走 欧拉先生 走过 的 路 , 比如 , 自己动手 建立一个 自然对数 体系, 如何 ?
这些 想法 也是 隐约 由来已久 , 但 写 这篇 文章 的 直接原因 是 今天 下午 在 民科吧 看到 这个 帖 《考大家一道数学题,请写出详细计算过程。》 https://tieba.baidu.com/p/7478296653 。
我在 民科吧 和 数学吧 经常 看到 学生党 提到 lim (1 + 1/n) ^ n , n -> 无穷 这个 极限, 据 (相关资料) 说, 这个 极限 就是 自然对数底 e ,也就是说, e = lim (1 + 1/n) ^ n , n -> 无穷 。
我想, 这个 极限 怎么求 先不管, 它 是 怎么来 的 ? 能不能 推导出来 ?
下午 的 时候, 我 试了一下, 还 真能 推导 出来 。
有关资料 显示, 自然对数 和 ʃ 1/x dx 这个 积分 有关, ʃ 1/x dx 我 在 《我决定在 反相吧 开展 一系列 的 趣味课堂, 来 普及 微积分 (4)》 https://www.cnblogs.com/KSongKing/p/12585020.html 的 第 24 讲 提过, 它 是 dy / dx = y 这个 微分方程 的 解 的 反函数 。
dy / dx = y 的 意义 是 y ′= y , 也就是 导数 和 原函数 一样, 是 同一个 函数, 也就是 存在一个 y = f ( x ) , 使得 f ‘ ( x ) = f ( x ) 。
我们现在 已经 知道 存在一个 指数函数 y = e^x , e 为常数 可以 满足 dy / dx = y , 求 e 。
因为 y = e^x 满足 dy / dx = y , 也就是
[ e ^ ( x + ⊿ x ) – e ^ x ] / ⊿ x = e ^ x , ⊿ x -> 0
e ^ ( x + ⊿ x ) – e ^ x = e ^ x * ⊿ x
e ^ x * e ^ ⊿ x – e ^ x = e ^ x * ⊿ x
e ^ ⊿ x – 1 = ⊿ x
e ^ ⊿ x = 1 + ⊿ x
e = ( 1 + ⊿ x ) ^ ( 1 / ⊿ x )
因为 ⊿ x 是 一个 无穷小, ⊿ x -> 0 , 因此, ⊿ x 也可以写成 1 / n , n ∈ N , n -> 无穷, 也就是 ⊿ x = 1 / n , n ∈ N , n-> 无穷 , 于是 ,
e = ( 1 + 1 / n ) ^ n , n -> 无穷
还有一个 说法, lim (1 + 1/n) ^ n , n -> 无穷 这个 极限 也叫做 “利滚利” , 啊,这 。
上文 说 “我们现在 已经 知道 存在一个 指数函数 y = e^x , e 为常数 可以 满足 dy / dx = y , 求 e 。” , 为什么知道 是 指数函数 而 不是 别的 函数 ? 为什么 确定 是 指数函数 而 不是 别的 函数 ?
这需要 一些 猜想 和 证明, 以后补上 。
接下来, lim (1 + 1/n) ^ n , n -> 无穷 这个 极限 怎么求 ?
有 泰勒级数 的 话, 求 e 很容易, 在 没有 泰勒级数 的 时代, 求 lim (1 + 1/n) ^ n , n -> 无穷 要 想些 办法, 可能 要 动用一些 “花招” 才行 。
有时候觉得, 泰勒级数 在 初等函数 领域 内 简直 是 杀手锏, 初等函数 领域 产生 的 超越数 π 、e 用 泰勒级数 表示 是 无往不利 。
哎 ? 初等函数 领域 还有 哪些 超越数 ? 按理, 应该 有 很多, 但印象中 据说 主要 就 π 、e 这两个 。
未完待续 。
说是 未完待续, 但 未来 还 续不续, 也是说不定的, 再说吧, 哈哈哈哈 。
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