Returns the value of the prime-counting function.
a matrix of floating point integers, the points where to evaluate the function, must be positive
a matrix of floating point integers, the function value
The prime-counting function of x is equal to the number of prime numbers less than or equal to x.
x = 1:60; y = number_primecount ( x ); plot(x,y,"bo")
http://en.wikipedia.org/wiki/Prime-counting_function