Two classes 
and 
of positive
integers are equinumerous if
 |
where 
is the number of integers in 
that are less than or equal to 
, and 
is the number of integers in 
that are less than or equal to 
. Equivalently, 
and 
have asymptotically equal counting functions.
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.
