Let 
and 
be two classes of positive
integers. Let 
be the number of integers in 
which are less than or equal to 
, and let 
be the number of integers in 
which are less than or equal to 
. Then if
 |
and 
are said to be equinumerous.
The four classes of primes 
, 
, 
, 
are equinumerous. Similarly, since 
and 
are both of the form 
, and 
and 
are both of the form 
, 
and 
are also equinumerous.
