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.
See also
Bertrand's Postulate, Prime Counting Function
Explore with Wolfram|Alpha
More things to try:
References
Shanks, D. Solved and Unsolved Problems in Number Theory, 4th ed. New York: Chelsea, pp. 21-22 and 31-32, 1993.
Referenced on Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Equinumerous." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Equinumerous.html