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 A057007 #18 Aug 09 2019 12:18:25 %S A057007 1,15,235,14120,1712845,371515454,127635996839,65417303558808, %T A057007 47718183491375681,47736450176936606373,63553335359217380046467, %U A057007 109839393591932655104274298,241347279732331127743077516005 %N A057007 Number of conjugacy classes of subgroups of index n in free group of rank 4. %D A057007 R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(c), pp. 76, 112. %H A057007 Vaclav Kotesovec, <a href="/A057007/b057007.txt">Table of n, a(n) for n = 1..60</a> %H A057007 J. H. Kwak and J. Lee, <a href="https://doi.org/10.1002/(SICI)1097-0118(199610)23:2<105::AID-JGT1>3.0.CO;2-X">Enumeration of connected graph coverings</a>, J. Graph Th., 23 (1996), 105-109. %H A057007 J. H. Kwak and J. Lee, <a href="http://com2mac.postech.ac.kr/resorce/Lecture_text.htm">Enumeration of graph coverings and surface branched coverings</a>, Lecture Note Series 1 (2001), Com^2MaC-KOSEF, Korea. See chapter 3. %H A057007 V. A. Liskovets, <a href="https://doi.org/10.1023/A:1005950823566">Reductive enumeration under mutually orthogonal group actions</a>, Acta Applic. Math., 52 (1998), 91-120. %H A057007 P. Vrana, <a href="https://doi.org/10.1088/1751-8113/44/22/225304">On the algebra of local unitary invariants of pure and mixed quantum states</a>, J. Phys A: Math. Theor. 44 (2011) 225304 doi:10.1088/1751-8113/44/22/225304, Table 2. %Y A057007 Cf. A057004-A057013. Inverse Euler transform of A160446. %K A057007 nonn %O A057007 1,2 %A A057007 _N. J. A. Sloane_, Sep 09 2000 %E A057007 More terms from Francisco Salinas (franciscodesalinas(AT)hotmail.com), Dec 25 2001