某顏冊頁見以下結果:
![johnmayhk-binomial-limit]()
(圖片來源:Mathematics)
在留言中提到
“…is a recently discovered beauty, probably in 2012…”
頓時有種叫人肅然起敬之感,但其實這不過是 M1,M2 的題目而已,同學可以先玩玩。
當然,我也要承認這是美麗的,起碼我從沒發現這個結果。
OK,做數時間:
對於足夠大的
,
![\frac{s_{n-1}s_{n+1}}{s_n^2} \frac{s_{n-1}s_{n+1}}{s_n^2}]()
![=\frac{C_0^{n-1}C_1^{n-1}C_2^{n-1}\dots C_{n-1}^{n-1}C_0^{n+1}C_1^{n+1}C_2^{n+1}\dots C_{n+1}^{n+1}}{C_0^nC_1^nC_2^n\dots C_n^nC_0^nC_1^nC_2^n\dots C_n^n} =\frac{C_0^{n-1}C_1^{n-1}C_2^{n-1}\dots C_{n-1}^{n-1}C_0^{n+1}C_1^{n+1}C_2^{n+1}\dots C_{n+1}^{n+1}}{C_0^nC_1^nC_2^n\dots C_n^nC_0^nC_1^nC_2^n\dots C_n^n}]()
![=(\frac{C_0^{n-1}}{C_0^n})(\frac{C_1^{n-1}}{C_1^n})(\frac{C_2^{n-1}}{C_2^n})\dots (\frac{C_{n-1}^{n-1}}{C_{n-1}^n})\frac{1}{C_n^n}(\frac{C_0^{n+1}}{C_0^n})(\frac{C_1^{n+1}}{C_1^n})(\frac{C_2^{n+1}}{C_2^n})\dots (\frac{C_n^{n+1}}{C_n^n})C_{n+1}^{n+1} =(\frac{C_0^{n-1}}{C_0^n})(\frac{C_1^{n-1}}{C_1^n})(\frac{C_2^{n-1}}{C_2^n})\dots (\frac{C_{n-1}^{n-1}}{C_{n-1}^n})\frac{1}{C_n^n}(\frac{C_0^{n+1}}{C_0^n})(\frac{C_1^{n+1}}{C_1^n})(\frac{C_2^{n+1}}{C_2^n})\dots (\frac{C_n^{n+1}}{C_n^n})C_{n+1}^{n+1}]()
易知
,故上式進一步化簡為
![(\frac{n-1}{n})(\frac{n-2}{n})\dots (\frac{2}{n})(\frac{1}{n})\times(\frac{n+1}{n})(\frac{n+1}{n-1})(\frac{n+1}{n-2})\dots (\frac{n+1}{2})(\frac{n+1}{1}) (\frac{n-1}{n})(\frac{n-2}{n})\dots (\frac{2}{n})(\frac{1}{n})\times(\frac{n+1}{n})(\frac{n+1}{n-1})(\frac{n+1}{n-2})\dots (\frac{n+1}{2})(\frac{n+1}{1})]()
![=\frac{(n-1)!}{n^{n-1}}\times \frac{(n+1)^n}{n!} =\frac{(n-1)!}{n^{n-1}}\times \frac{(n+1)^n}{n!}]()
![=(1+\frac{1}{n})^n =(1+\frac{1}{n})^n]()
於是
![\displaystyle \lim_{n\rightarrow \infty}\frac{s_{n-1}s_{n+1}}{s_n^2} \displaystyle \lim_{n\rightarrow \infty}\frac{s_{n-1}s_{n+1}}{s_n^2}]()
![=\displaystyle \lim_{n\rightarrow \infty}(1+\frac{1}{n})^n =\displaystyle \lim_{n\rightarrow \infty}(1+\frac{1}{n})^n]()
![=e =e]()
有關涉及二項係數的極限,很久以前在純數科考試卷擬過,現在修 M1,M2 的同學已無緣享用,在這貼貼算了:
Evaluate
![\displaystyle \lim_{n\rightarrow \infty}\sqrt[n]{\frac{C_n^{3n}}{C_n^{2n}}} \displaystyle \lim_{n\rightarrow \infty}\sqrt[n]{\frac{C_n^{3n}}{C_n^{2n}}}]()
答案是
,極不漂亮。