This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A090406 #19 Feb 16 2025 08:32:51 %S A090406 2,2,1,2,1,1,0,1,1,2,1,1,0,1,1,2,1,1,0,1,1,1,0,0,0,1,1,2,1,1,0,0,0,1, %T A090406 1,1,0,1,1,2,1,1,0,1,1,1,0,0,0,1,1,1,0,0,0,1,1,2,1,1,0,0,0,1,1,1,0,1, %U A090406 1,2,1,1,0,0,0,1,1,1,0,1,1,1,0,0,0,1,1,1,0,0,0,0,0,1,1,1,0,1,1,2,1,1 %N A090406 a(n) = PrimePi(n+3) - PrimePi(n). %C A090406 a(n) <= 2, with equality if and only if n=1 or n+1 is in A001359. - _Robert Israel_, Aug 14 2015 %H A090406 Robert Israel, <a href="/A090406/b090406.txt">Table of n, a(n) for n = 1..10000</a> %H A090406 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Hardy-LittlewoodConjectures.html">Hardy-Littlewood Conjectures</a> %F A090406 a(n) = A010051(n+1) + A010051(n+2) + A010051(n+3). - _Robert Israel_, Aug 14 2015 %p A090406 B:= [seq(numtheory:-pi(n),n=1..103)]: %p A090406 B[4..-1] - B[1..-4]; # _Robert Israel_, Aug 14 2015 %t A090406 Last[#]-First[#]&/@Partition[PrimePi[Range[110]],4,1] (* _Harvey P. Dale_, Feb 24 2013 *) %Y A090406 Cf. A000720, A001359, A010051, A080545, A090405. %K A090406 nonn %O A090406 1,1 %A A090406 _Eric W. Weisstein_, Nov 29 2003