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 A057012 #8 Aug 10 2019 04:22:01 %S A057012 1,624,504243,371515454,268530771271,193466859054994, %T A057012 139311082645798043,100305771690618678654,72220370631411094037391, %U A057012 51998692654400641678907114,37439061807069469917891862243 %N A057012 Number of conjugacy classes of subgroups of index 6 in free group of rank n. %D A057012 R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(c), pp. 76, 112. %H A057012 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 A057012 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 A057012 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. %o A057012 (PARI) a(n)=if(n<0,0,n--;720^n-120^n+24^n+18^n-16^n+12^n*2-36^n/2-9^n/2+8^n*2/3-6^n/2-4^n*3/2-3^n/2+2^n*5/6) %Y A057012 Cf. A057004-A057013. %K A057012 nonn %O A057012 1,2 %A A057012 _N. J. A. Sloane_, Sep 09 2000 %E A057012 More terms from Francisco Salinas (franciscodesalinas(AT)hotmail.com), Dec 25 2001