斐波那契数列
来源:https://www.bilibili.com/video/BV1XV41127JW
1、什么是斐波那契数列?
斐波那契数列就是从第3项开始,每一项都等于前两项之和。
例子:1,1,2,3,5,8,13,21,34,55,89 ......
2、记忆点
1)所有数据相加
\[S_n = a_{n+2} - a_2
\]
2)奇、偶下标相加
\[奇数:a_1+a_3+a_5+a_7+\cdots+a_{2n-1}=S_{2n-2}+a_1=a_{2n}-a_2+a_1\\
偶数:a_2 + a_4 + a_6 + a_8 + \cdots + a_{2n} = S_{2n-1} - a_1 + a_2 = a_{2n+1} - a_1
\]
3)任意连续三项
\[a_{n+1}a_{n-1}-a_n^2 = (-1)^n
\]
4)3.1
\[a_n^2 = a_{n+1}a_n- a_na_{n-1}
\]
5)余数列的周期性
被2除的与数列周期为3: 1,1,0....
被4除的与数列周期为6: 1,1,2,3,1,0....
被3除的与数列周期为8: 1,1,2,0,2,2,1,0....
3、常用的性质
性质一
\[S_n = a_{n+2} - a_2
\]
推导:
\[\begin{cases}
&a_{n+2}=a_{n+1} + a_n \\
&a_{n+1}=a_n + a_{n-1}\\
&a_{a_n}=a_{n-1} + a_{n-2}\\
&\cdots\\
&a_4=a_3+a_2\\
&a_3=a_2+a_1\\
\end{cases}
\]
将上述的式子相加就会得到:
\[a_{n+2}= (a_n+a_{n-1}+a_{n-2}+\cdots+a_2+a_1)+a_2\\
↓\\
a_{n+2} = S_n + a_2\\
↓\\
S_n = a_{n+2} - a_2
\]
性质二
奇数项之和:
\[a_1+a_3+a_5+a_7+\cdots+a_{2n-1}=S_{2n-2}+a_1=a_{2n}-a_2+a_1\\
当a1=a2=1时↓\\
a_1+a_3+a_5+a_7+\cdots+a_{2n-1}=S_{2n-2}+a_1=a_{2n}
\]
将除了第一项的数据进行拆分
\[a_1+[(a_1+a_2)+(a_3+a_4)+(a_5+a_6)+\cdots+(a_{2n-3}+a_{2n-2})]=a_1+S_{2n-2}\\
↓↓↓S_n=a_{n+2}-a_2(性质一的结论)↓↓↓\\
a_1+S_{2n-2}=a_1+[a_{2n}-a_2]
\]
偶数项之和:
\[a_2 + a_4 + a_6 + a_8 + \cdots + a_{2n} = S_{2n-1} - a_1 + a_2 = a_{2n+1} - a_1\\
当a1=a2=1时↓\\
a_2 + a_4 + a_6 + a_8 + \cdots + a_{2n} = S_{2n-1} = a_{2n+1} - 1
\]
将除了第一项的数据进行拆分
\[a_2+[(a_2+a_3)+(a_4+a_5)+(a_6+a_7)+\cdots+(a_{2n-2}+a_{2n-1})]+a_1-a_1 = a_2+S_{2n-1}-a_1\\
↓↓↓S_n=a_{n+2}-a_2(性质一的结论)↓↓↓\\
a_2+S_{2n-1}-a_1 =a_{2n+1} - a_1
\]
性质三
\[a_{n+1}a_{n-1}-a_n^2 = (-1)^n
\]
性质四
\[a_n^2 = a_{n+1}a_n- a_na_{n-1}
\]
推导
\[a_{n+1}a_n- a_na_{n-1}\\
=(a_n+a_{n-1})a_n - a_na_{n-1}\\
=a_n^2+ a_na_{n-1}- a_na_{n-1}\\
=a_n^2
\]
性质五
被2除的与数列周期为3: 1,1,0....
被4除的与数列周期为6: 1,1,2,3,1,0....
被3除的与数列周期为8: 1,1,2,0,2,2,1,0....