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 A053683 #16 Feb 16 2025 05:18:28 %S A053683 2,5,16,34,91,130,228,289,430,695,799,1150,1418,1566,1880,2400,2994, %T A053683 3202,3880,4366,4624,5430,6003,6921,8246,8949,9315,10068,10458,11246, %U A053683 14252,15185,16628,17131,19712,20254,21920,23654,24846,26688,28605 %N A053683 Number of nonprimes <= prime(n)^2. %H A053683 Amiram Eldar, <a href="/A053683/b053683.txt">Table of n, a(n) for n = 1..10000</a> %F A053683 a(n) = A000040(n)^2 - A000879(n). - _Jean-Christophe Hervé_, Oct 27 2013 %e A053683 Up to 25 = prime(3)^2, the 16 nonprimes are 1, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25. %t A053683 a[n_] := Prime[n]^2 - PrimePi[Prime[n]^2]; Array[a, 50] (* _Amiram Eldar_, Feb 16 2025 *) %o A053683 (PARI) a(n) = prime(n)^2 - primepi(prime(n)^2); \\ _Michel Marcus_, Oct 28 2013 %Y A053683 Cf. A000879. %K A053683 easy,nonn %O A053683 1,1 %A A053683 _Labos Elemer_, Feb 16 2000