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Primes p:  0  

The result of p: i is the ith prime. For example:
   p: 0
2
    

   p: i. 15
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47
The inverse of p: is the number of primes less than the argument, often denoted by p(n): 
   pi=: p:^:_1
   pi i. 15
0 0 0 1 2 2 3 3 4 4 4 4 5 5 6

   (] , pi ,: p:@pi) i.15
0 1 2 3 4 5 6 7  8  9 10 11 12 13 14
0 0 0 1 2 2 3 3  4  4  4  4  5  5  6
2 2 2 3 5 5 7 7 11 11 11 11 13 13 17

   y=: (2^31)-1

   ]a=: pi y
105097564

   ]b=: p: a
2147483647

   b=y
1


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