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 A072613 Number of numbers of the form p*q (p, q distinct primes) less than or equal to n. 3
 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 4, 4, 4, 4, 4, 4, 5, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 8, 9, 10, 10, 10, 11, 12, 12, 12, 12, 12, 12, 12, 13, 13, 13, 13, 13, 14, 14, 14, 14, 15, 15, 16, 17, 17, 17, 17, 18, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 20, 21, 21, 21, 22, 22, 22, 22 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,10 COMMENTS There was an old comment here that said a(n) was equal to A070548(n) - 1, but this is false (e.g. at n=210). - N. J. A. Sloane, Sep 10 2008 REFERENCES G. Tenenbaum. Introduction to Analytic and Probabilistic Number Theory. (Cambridge Studies in Advanced Mathematics 1995). LINKS N. J. A. Sloane, Table of n, a(n) for n = 1..10000 Weisstein, Eric W., Semiprime. FORMULA a(n)=\sum_{p= n then break; fi; for j from i+1 to n do q:=ithprime(j); if p*q > n then break; fi; c:=c+1; od: od; RETURN(c); end; - N. J. A. Sloane, Sep 10 2008 MATHEMATICA fPi[n_] := Sum[ PrimePi[n/ Prime@i] - i, {i, PrimePi@ Sqrt@ n}]; Array[ fPi, 81] (* Robert G. Wilson v, Jul 22 2008 *) PROG (PARI) a(n)=sum(k=1, n, if(abs(omega(k)-2)+(1-issquarefree(k)), 0, 1)) CROSSREFS Cf. A072000. Sequence in context: A069623 A076411 A217038 * A029551 A171482 A132015 Adjacent sequences:  A072610 A072611 A072612 * A072614 A072615 A072616 KEYWORD easy,nonn AUTHOR Benoit Cloitre, Aug 11 2002 STATUS approved

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Last modified June 19 07:18 EDT 2013. Contains 226399 sequences.