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A122121
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Number of primes <= 10^(n/2).
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15
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0, 2, 4, 11, 25, 65, 168, 446, 1229, 3401, 9592, 27293, 78498, 227647, 664579, 1951957, 5761455, 17082666, 50847534, 151876932, 455052511, 1367199811, 4118054813, 12431880460, 37607912018, 113983535775, 346065536839, 1052370166553, 3204941750802
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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FORMULA
| a(2n)=A006880(n). - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 13 2007
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EXAMPLE
| a(3) = 11: sqrt(1000) = 31.62277660..., pi(31) = 11.
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MATHEMATICA
| a={}; For[n=0, n<=27, n++, AppendTo[a, PrimePi[10^(n/2)]]]; Print[a]; [From John W. Layman (layman(AT)math.vt.edu), Mar 12 2010]
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PROG
| (PARI) { a= 0; n= 1; p=2 ; while(1, a++ ; pnext =nextprime(p+1) ; if( p^2 <= 10^n && pnext^2>10^n, print(a) ; n++ ; ) ; p=pnext ; ) ; } - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 13 2007
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CROSSREFS
| Cf. A000720, A006880.
Sequence in context: A100550 A071973 A086424 * A080009 A159336 A151257
Adjacent sequences: A122118 A122119 A122120 * A122122 A122123 A122124
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), based on a suggestion from Klaus Kastberg (Kastberg(AT)aapt.net.au), Oct 17 2006
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EXTENSIONS
| More terms from R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jan 13 2007
a(0) - a(17) confirmed, and a(18) - a(26) added using Mathematica, by John W. Layman (layman(AT)math.vt.edu), Mar 12 2010
a(27) and a(28) added using Mathematica, by David Baugh (82to79(AT)gmail.com), Oct 06 2011
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