$\left[\begin{array}{cc} F_{n-1}& F_{n} \\ F_{n}& F_{n+1} \end{array}\right]$

## 两个引理

$\frac{ax+b}{cx+d}=x$

## 定理的证明

1. Joseph A. Gallian, Contemporary Abstract Algebra, 5th ed., Houghton Mifﬂin, Boston, MA, 2002.
2. R. C. Lyndon and J. L. Ullman, Groups of elliptic linear fractional transformations, Proc. Amer. Math. Soc. 18 (1967), 1119–1124.
3. Dresden, Gregory P. "There are only nine finite groups of fractional linear transformations with integer coefficients." Mathematics Magazine (2004): 211-218.