## 斐波那契算法

$$f(n) = \frac{1}{{\sqrt 5 }}\left[ {{{\left( {\frac{1}{2}\left( {\sqrt 5 + 1} \right)} \right)}^n} - {{\left( {\frac{2}{{\sqrt 5 + 1}}} \right)}^n}\cos \left( {\pi n} \right)} \right]$$

## 九宫格密码盘

$$\begin{array}{*{20}{c}} A&B \\ C&D \end{array}$$

$$\mathop \sum \limits_{k = 2}^n \frac{{n!}}{{\left( {n - k} \right)!}} = {\text{e}}\Gamma \left( {n + 1,1} \right) - (1 + n)$$

## 数字黑洞III:数位黑洞

A factorion is a natural number that equals the sum of the factorials of its decimal digits.