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 A027840 #16 Jul 08 2025 17:55:48 %S A027840 1,15,220,5275,151086,6605004,362069912,26370058035,2384037107365, %T A027840 264380945199210,35133143655934644,5515729438742221708, %U A027840 1009373492449379367974,212997911074525038601560,51337590023913924398371080,14016616814674335739387516003 %N A027840 Number of subgroups of index n in fundamental group of a certain fiber space. %H A027840 Sean A. Irvine, <a href="https://github.com/archmageirvine/joeis/blob/master/src/irvine/oeis/a027/A027840.java">Java program</a> (github). %H A027840 V. A. Liskovets and A. Mednykh, <a href="https://doi.org/10.1080/00927870008826924">Enumeration of subgroups in the fundamental groups of orientable circle bundles over surfaces</a>, Commun. in Algebra, 28, No. 4 (2000), 1717-1738. %Y A027840 Cf. A027838, A027839, A027841, A060240. %K A027840 nonn %O A027840 1,2 %A A027840 _N. J. A. Sloane_ %E A027840 Third term corrected from 240 to 220, Aug 15 1999 %E A027840 More terms from _Sean A. Irvine_, Dec 07 2019