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.

**References**

Shanks, D. *Solved and Unsolved Problems in Number Theory, 4th ed.* New York: Chelsea, pp. 21-22 and 31-32, 1993.

© 1996-9

1999-05-25