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 A122121 Number of primes <= 10^(n/2). 17
 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, 9773865306521 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS David Baugh, Table of n, a(n) for n = 0..52 FORMULA a(2n) = A006880(n). - R. J. Mathar, Jan 13 2007 EXAMPLE a(3) = 11: sqrt(1000) = 31.62277660..., pi(31) = 11. MATHEMATICA a={}; For[n=0, n<=27, n++, AppendTo[a, PrimePi[10^(n/2)]]]; Print[a]; (* John W. Layman, Mar 12 2010 *) 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, Jan 13 2007 CROSSREFS Cf. A000720, A006880. Sequence in context: A329707 A071973 A086424 * A080009 A159336 A240571 Adjacent sequences:  A122118 A122119 A122120 * A122122 A122123 A122124 KEYWORD nonn AUTHOR N. J. A. Sloane, based on a suggestion from Klaus Kastberg (Kastberg(AT)aapt.net.au), Oct 17 2006 EXTENSIONS More terms from R. J. Mathar, Jan 13 2007 a(0)-a(17) confirmed, and a(18)-a(26) added using Mathematica, by John W. Layman, Mar 12 2010 a(27) and a(28) added using Mathematica, by David Baugh, Oct 06 2011 a(29) from Donovan Johnson, Mar 12 2013 a(30)-a(46) added using Kim Walisch's primecount program, by David Baugh, Feb 10 2015 a(47)-a(52) from David Baugh using Kim Walisch's primecount program, Jun 19, 2016 STATUS approved

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Last modified May 16 11:20 EDT 2021. Contains 343942 sequences. (Running on oeis4.)