Suppose $f$ is a real valued function with domain a subset of
real numbers, $D$.

Definition: $f$ is a **linear fractional function**
if **there a numbers $a,b,c$, and $d$ so that for any $x \in
D$, $f(x) =\frac {ax+b}{cx +d}$**

**Comment: **An important result about linear fractional
functions shows the relation of these functions to the core negative
power function $R(x)=\frac 1 x$.

Theorem LFF.COMP.