The Fibonacci Numbers
One special recursively defined function, which has no simple explicit 
definition, yields the Fibonacci numbers:

The values of this function are:
| f (1) | = | 1 | 
  | 
| f (2) | = | 1 | 
  | 
| f (3) | = | 1 + 1 = 2 | 
  | 
| f (4) | = | 1 + 2 = 3 | 
  | 
| f (5) | = | 2 + 3 = 5 | 
  | 
| f (6) | = | 3 + 5 = 8 | 
  | 
| f (7) | = | 5 + 8 = 13 | 
  | 
| f (8) | = | 8 + 13 = 21 | 
  | 
| f (9) | = | 13 + 21 = 34 | 
  | 
| 
 |   |   | 
  | 
 
Thus, the sequence of Fibonacci numbers is 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 
.  These numbers have many interesting properties that will be studied in 
higher math.  They recur often in mathematics and even in nature.