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 A211178 #24 Dec 04 2020 11:50:43
%S A211178 1,1,2,1,4,4,12,3,6,12,60,30,60,20,40,20,80,240,720,720,720,144,1584,
%T A211178 1584,7920,7920,7920,7920,55440,55440,11088,5544,27720,55440,55440,
%U A211178 55440,55440,6160,18480,2310,9240,9240,3080,3080,1155,210,2415,38640,5520,5520
%N A211178 Denominator of Sum_{k=1..n}(-1)^k/phi(k), where phi = A000010.
%H A211178 Amiram Eldar, <a href="/A211178/b211178.txt">Table of n, a(n) for n = 1..10000</a>
%H A211178 Olivier Bordellès and Benoit Cloitre, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL16/Bordelles/bord14.html">An Alternating Sum Involving the Reciprocal of Certain Multiplicative Functions</a>, J. Int. Seq., Vol. 16 (2013) Article #13.6.3.
%F A211178 A211177(n)/a(n) = c*log(n) + O(1) with a suitable constant c (see ref).
%F A211178 The constant above is c = zeta(2)*zeta(3)/(3*zeta(6)) = (1/3) * A082695. - _Amiram Eldar_, Nov 20 2020
%t A211178 Denominator @ Accumulate[Table[(-1)^k/EulerPhi[k], {k, 1, 50}]] (* _Amiram Eldar_, Nov 20 2020 *)
%o A211178 (PARI) a(n)=denominator(sum(k=1, n, (-1)^k/eulerphi(k)))
%Y A211178 Cf. A000010, A082695, A211177 (numerators).
%K A211178 nonn,frac
%O A211178 1,3
%A A211178 _Benoit Cloitre_, Feb 01 2013
%E A211178 More terms from _Amiram Eldar_, Nov 20 2020