A notation for the [[Secant Line]] slope in a sequence.
$\huge \begin{align}
f[x_{i}] &= y_{i} \\
f[x_{i},x_{i+1}] &= \frac{f[x_{i}]-f[x_{i+1}]}{x_{i}-x_{i+1}} \\
f[x_{i},x_{i+1},x_{i+2}] &=
\frac{f[x_{i},x_{i+1}] - f[x_{i+1},x_{i+2}]}{
x_{i} - x_{i+2} } \\
\end{align}$
In General,
$\huge
f[x_{i}, \dots, x_{j}] =
\frac{
f[x_{i}, \dots,x_{j-1}] - f[x_{i+1},\dots,x_{j}]
}{
x_{i}-x_{j}
}
$