**
Combination Function
** -

*C*(*n*, *k*) =
.

**
Discrete Function
** -
A function that is defined only for a set of numbers that can be listed, such as
the set of whole numbers or the set of
integers.

**
Explicit Definition
** -
A definition of a function by a formula in terms of the variable.

**
Factorial Function
** -
*F*(*n*) = *n*(*n* - 1)(*n* - 2)(*n* - 3)^{ ... }(2)(1)

where
*n*
is a non-negative integer.

We define
*F*(0) = 1
and
*F*(1) = 1
.

**
Fibonacci Sequence
** -
The sequence 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ..., where each value is the sum
of the preceding two values. Defined recursively by:

**
Permutation Function
** -

*P*(*n*, *k*) =
**
Recursive Definition
** -
A definition of a function in terms of the same function of a smaller variable.