Combinatorics: Make combinatorial statistics calculations

Hello!
This class contains some useful statistical functions to calculate dispositions
and combinations of objects.
An interesting use of it could be  to find the best path,
both if you have to visit a certain number of cities  and if you have to touch a
certain number of nodes in a network.
Of course, developping similar functions inside a specific database (using pl/sql  for Oracle, Transact SQl for SQLServer, plpgsql for Postgresql)
 is probably more efficient, but this approach is database independent.
I have done some tests and they seem to work. 
If you find any errors or  have some suggestions please contact me.
Have a nice day
:-)

p.s. The main class is Combinatorics. 
The class Geo extends Combinatorics and  was built only to be used in the example.