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 A291330 #14 Jan 16 2025 02:39:14
%S A291330 0,2,0,4,4,6,4,8,8,10,8,12,12,14,12,16,16,18,16,20,20,22,20,24,24,26,
%T A291330 24,28,28,30,28,32,32,34,32,36,36,38,36,40,40,42,40,44,44,46,44,48,48,
%U A291330 50,48,52,52,54,52,56,56,58,56,60,60,62,60,64,64,66,64,68,68
%N A291330 The arithmetic function v_4(n,1).
%D A291330 J. Butterworth, Examining the arithmetic function v_g(n,h). Research Papers in Mathematics, B. Bajnok, ed., Gettysburg College, Vol. 8 (2008).
%H A291330 Bela Bajnok, <a href="https://arxiv.org/abs/1705.07444">Additive Combinatorics: A Menu of Research Problems</a>, arXiv:1705.07444 [math.NT], May 2017. See Table in Section 1.6.1.
%F A291330 a(n) = n - gcd(n,4) = n - A109008(n). - _Ridouane Oudra_, Dec 15 2024
%F A291330 Sum_{n>=5} (-1)^n/a(n) = (1 - log(2))/2. - _Amiram Eldar_, Jan 15 2025
%p A291330 seq(n-gcd(n,4), n=2..100); # _Ridouane Oudra_, Dec 15 2024
%t A291330 v[g_, n_, h_] := (d = Divisors[n]; Max[(Floor[(d - 1 - GCD[d, g])/h] + 1)*n/d]); Table[v[4, n, 1], {n, 2, 70}]
%o A291330 (PARI) /* Adapted from Mathematica program */
%o A291330 v(g, n, h) = my(d=divisors(n)); for(k=1, #d, d[k]=floor(((d[k]-1-gcd(d[k], g))/h) + 1)*n/d[k]); vecmax(d)
%o A291330 a(n) = v(4, n, 1) \\ _Felix Fröhlich_, Aug 22 2017
%Y A291330 Cf. A289435, A289436, A289437, A289438, A289439, A289440, A289441.
%Y A291330 Cf. A109008.
%K A291330 nonn
%O A291330 2,2
%A A291330 _Robert Price_, Aug 22 2017