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%I #27 Feb 23 2014 02:50:33
%S 0,0,0,1,1,1,1,2,2,3,3,3,3,3,3,4,4,4,3,4,4,5,5,5,6,6,6,7,6,6,6,7,7,7,
%T 8,8,8,9,9,9,9,9,9,9,9,10,9,9,9,10,10,11,11,11,12,13,13,14,13,13,12,
%U 12,12,12,13,13,13,13,13,14,14,14,13,13,13,14,15,15,14,15,15,15,15,15
%N Number of primes p such that n < p < 2n-2.
%C a(n) > 0 for n > 3 by Bertrand's postulate (and Chebyshev's proof of 1852). - _Jonathan Vos Post_, Aug 08 2013
%H J. Sondow and E. Weisstein, <a href="http://mathworld.wolfram.com/BertrandsPostulate.html">Bertrand's Postulate</a>, World of Mathematics
%H M. Tchebichef, <a href="http://gallica.bnf.fr/ark:/12148/bpt6k163969/f374.image.r=periodiques.langFR">Memoire sur les nombres premiers</a>, J. Math. Pures Appliq. 17 (1852) 366.
%e a(19) = 3, the first value smaller than a previous value, because the only primes between 19 and 2 * 19 - 2 = 36 are {23,29,31}. - _Jonathan Vos Post_, Aug 08 2013
%t a[n_] := PrimePi[2n - 2] - PrimePi[n]; a[1] = 0; Table[a[n], {n, 1, 100}] (* _Jean-François Alcover_, Oct 31 2012 *)
%Y Cf. A060715.
%K nonn
%O 1,8
%A _Eric W. Weisstein_, Nov 05 2002
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