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 A275495 #28 May 28 2021 03:23:58
%S A275495 0,1,2,2,3,4,5,4,6,7,8,7,8,9,12,10,11,12,13,12,15,16,17,14,16,17,20,
%T A275495 19,20,21,22,19,22,23,26,24,25,26,29,26,27,28,29,28,33,34,35,30,32,33,
%U A275495 36,35,36,37,40,37,40,41,42,39,40,41,46,42,45,46,47,46,49
%N A275495 a(n) = Sum_{k=2..n} floor(n/k) - 2*floor(n/(2*k)).
%H A275495 Charles R Greathouse IV, <a href="/A275495/b275495.txt">Table of n, a(n) for n = 1..10000</a>
%F A275495 a(n) = Sum_{i=1..n} floor((n-i)/i)*(-1)^(i+1). - _Wesley Ivan Hurt_, Sep 13 2017
%F A275495 a(n) = Sum_{i=2..n} (floor(n/i) mod 2) = A059851(n) - (n mod 2). - _Ridouane Oudra_, Oct 20 2019
%F A275495 a(n) ~ log(2) * n. - _Vaclav Kotesovec_, May 28 2021
%p A275495 seq(add(floor(n/k)-2*floor(n/(2*k)), k=2..n), n=1..60); # _Ridouane Oudra_, Oct 20 2019
%t A275495 Table[Sum[Floor[n/k] - 2*Floor[n/(2*k)], {k, 2, n}], {n, 1, 50}] (* _G. C. Greubel_, Jul 30 2016 *)
%o A275495 (Sage)
%o A275495 [sum([floor(n/k) - 2*floor(n/(2*k)) for k in (2..n)]) for n in (1..69)]
%o A275495 (PARI) a(n)=sum(k=2,n,n\k) - 2*sum(k=2,n\2,n\(2*k)) \\ _Charles R Greathouse IV_, Jul 30 2016
%Y A275495 Cf. A002541, row sums of A275510, A059851.
%K A275495 nonn,easy
%O A275495 1,3
%A A275495 _Peter Luschny_, Jul 30 2016