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 A051421 #28 Feb 20 2023 22:08:38 %S A051421 1,2,12,185,8990,1505939,875542491,1789247738400,13018820342147705, %T A051421 341188114831706152794,32520852428719860881939391, %U A051421 11366533535523591133597276823755,14669006027884671703581740693080811331,70315546525961698601351615055416574931833334 %N A051421 Number of digraphs on n unlabeled nodes with a sink (or, with a source). %C A051421 Here a sink is defined to be a node to which all other nodes have a directed path. - _Andrew Howroyd_, Dec 27 2021 %D A051421 F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 218 (incorrect version). %D A051421 Ronald C. Read, email to _N. J. A. Sloane_, 28 August, 2000. %H A051421 Andrew Howroyd, <a href="/A051421/b051421.txt">Table of n, a(n) for n = 1..50</a> %o A051421 (PARI) \\ See PARI link in A350794 for program code. %o A051421 A051421seq(15) \\ _Andrew Howroyd_, Jan 21 2022 %Y A051421 The labeled case is A003028. %Y A051421 Row sums of A057277. %Y A051421 Cf. A003085, A035512, A003088, A049531, A350415, A350360, A350794. %K A051421 nonn,nice %O A051421 1,2 %A A051421 _Vladeta Jovovic_ %E A051421 a(6) corrected and a(7) from _Sean A. Irvine_, Sep 11 2021 %E A051421 a(0)=1 removed and terms a(8) and beyond from _Andrew Howroyd_, Jan 21 2022