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Counting RSA-integers

In the RSA cryptosystem integers of the form n=p.q with p and q primes of comparable size (`RSA-integers&#39;) play an important role. It is a folklore result of cryptographers that C_r(x), the number of integers n<=x that are of the form n=pq with p and q primes such that p<q<rp, is for fixed r>1 asymptotically equal to c_r*x*log^{-2}x for some constant c_r>0. Here we prove this and show that c_r=2log r.