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A175613
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Number of semiprimes <= 2^prime(n).
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0
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1, 2, 10, 42, 589, 2186, 30253, 113307, 1608668, 88157689, 336717854, 19015826478, 282528883551, 1091574618496, 16360940729894
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OFFSET
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1,2
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LINKS
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FORMULA
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EXAMPLE
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a(2)=2 because first 2 semiprimes are 4, 6 both <2^prime(2)=8.
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MATHEMATICA
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(* First run program given in A072000 to define the SemiPrimePi function *) Table[SemiPrimePi[2^Prime[n]], {n, 10}](* Alonso del Arte, Dec 10 2010 *)
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PROG
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(PARI) a(n)=my(N=2^prime(n), s, i); forprime(p=2, sqrtint(N), s+=primepi(N\p); i++); s - i * (i-1)/2 \\ Charles R Greathouse IV, Apr 25 2016
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CROSSREFS
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KEYWORD
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nonn,less
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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