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A125527
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Number of semiprimes <= 2^n.
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10
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0, 1, 2, 6, 10, 22, 42, 82, 157, 304, 589, 1124, 2186, 4192, 8110, 15658, 30253, 58546, 113307, 219759, 426180, 827702, 1608668, 3129211, 6091437, 11868599, 23140878, 45150717, 88157689, 172235073, 336717854, 658662065, 1289149627, 2524532330
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OFFSET
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1,3
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LINKS
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Eric Weisstein's World of Mathematics, Semiprime.
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FORMULA
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MATHEMATICA
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SemiPrimePi[n_] := Sum[ PrimePi[ n/Prime@i] - i + 1, {i, PrimePi@ Sqrt@n}]; Table[ SemiPrimePi[2^n], {n, 47}]
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PROG
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(PARI) a(n)=my(s, i, N=2^n); forprime(p=2, sqrtint(N), s+=primepi(N\p); i++); s - i * (i-1)/2 \\ Charles R Greathouse IV, May 12 2013
(Perl) use ntheory ":all"; print "$_ ", semiprime_count(1 << $_), "\n" for 1..48; # Dana Jacobsen, Sep 10 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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