 *  Execution:    java -Xmx1100m PrimeSieve N
 *  
 *  Computes the number of primes less than or equal to N using
 *  the Sieve of Eratosthenes.
 *
 *  % java PrimeSieve 25
 *  The number of primes <= 25 is 9
 *
 *  % java PrimeSieve 100
 *  The number of primes <= 100 is 25
 *
 *  % java -Xmx512m PrimeSieve 100000000
 *  The number of primes <= 100000000 is 5761455
 *
 *  % java PrimeSieve -Xmx1100m 1000000000 
 *  The number of primes <= 1000000000 is 50847534