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 A064427 #36 Feb 16 2025 08:32:45
%S A064427 1,2,4,6,7,9,10,12,13,14,15,17,18,20,21,22,23,25,26,28,29,30,31,33,34,
%T A064427 35,36,37,38,40,41,43,44,45,46,47,48,50,51,52,53,55,56,58,59,60,61,63,
%U A064427 64,65,66,67,68,70,71,72,73,74,75,77,78,80,81,82
%N A064427 a(n) = n + (number of primes < n).
%C A064427 From _Jaroslav Krizek_, Dec 10 2009: (Start)
%C A064427 Complement of A014688.
%C A064427 Numbers that are not the sum of k and the k-th prime for any k >= 1. (End)
%H A064427 Reinhard Zumkeller, <a href="/A064427/b064427.txt">Table of n, a(n) for n = 1..10000</a>
%H A064427 Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_821.htm">Puzzle 821. Prime numbers and complementary sequences</a>, The Prime Puzzles & Problems Connection.
%H A064427 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PrimeCountingFunction.html">Prime Counting Function</a>.
%H A064427 Wikipedia, <a href="http://en.wikipedia.org/wiki/Prime-counting_function">Prime-counting function</a>.
%F A064427 For n > 1: a(n) = n + A000720(n-1).
%t A064427 a[n_] := PrimePi[a[n-1]]+n; a[1]=1
%t A064427 Table[PrimePi[n-1]+n,{n,60}] (* _Harvey P. Dale_, Apr 03 2015 *)
%o A064427 (Haskell)
%o A064427 a064427 1 = 1
%o A064427 a064427 n = a000720 (n - 1) + toInteger n
%o A064427 -- _Reinhard Zumkeller_, Apr 17 2012
%o A064427 (PARI) a(n) = if (n==1, 1, primepi(n-1)+n); \\ _Michel Marcus_, Feb 13 2016
%o A064427 (Magma) [1] cat [#PrimesUpTo(n-1)+n: n in [2..100]]; // _Vincenzo Librandi_, Feb 13 2016
%Y A064427 Cf. A000720, A014688, A095117.
%K A064427 nonn
%O A064427 1,2
%A A064427 _Santi Spadaro_, Sep 30 2001
%E A064427 Definition improved by _Reinhard Zumkeller_, Apr 16 2012
%E A064427 Edited by _Jon E. Schoenfield_, Nov 24 2023