A Friedman number is a positive integer expressible nontrivially using all of its own decimal digits exactly once, together with plus, minus, times, division, exponentiation, parentheses, and concatenation (Friedman 2000). The digits may be rearranged, but merely writing the original number is excluded. For example,
 |
and other examples are 
,
, and 
.
Inserting decimal points or using additional operations such as square root or factorial does not belong to the standard definition (Friedman 2000).
