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 A275464 #20 Feb 23 2018 23:39:16
%S A275464 0,0,0,3,1,1,6,2,2,9,3,10,12,5,5,15,15,7,18,8,8,21,9,21,24,11,25,27,
%T A275464 13,13,30,30,15,33,16,16,36,35,18,39,19,40,42,21,42,45,45,24,48,25,25,
%U A275464 51,26,26,54,27,55,57,56,56,60,60,33,63,34,63,66,36,36,69,67,70,72,40,40,75,75,42,78,77,44,81
%N A275464 a(n) = n - A038802(n).
%H A275464 Raphael Ranna, <a href="/A275464/b275464.txt">Table of n, a(n) for n = 1..1000</a>
%F A275464 a(n) = n - (A049084(A020639(2n+1)) - 1).
%F A275464 a(n) = n + O(n/log n). - _Charles R Greathouse IV_, Jul 29 2016
%e A275464 9 = (2^0)*(3^2), resulting in 1 zero. So, a(4) = 4 - 1 = 3.
%t A275464 Table[f = FactorInteger[2 n + 1]; n - (PrimePi[f[[1, 1]]] - 1), {n, 100}]
%o A275464 (PARI) lpf(n)=factor(n)[1,1]
%o A275464 a(n)=n - primepi(lpf(2*n+1)) + 1 \\ _Charles R Greathouse IV_, Jul 29 2016
%Y A275464 Cf. A038802.
%K A275464 nonn
%O A275464 1,4
%A A275464 _Raphael Ranna_, Jul 28 2016
%E A275464 Duplicate a(44)-a(45) removed by _Andrew Howroyd_, Feb 23 2018