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 A053400 #15 Jul 10 2024 02:59:16 %S A053400 1,4,20,276,10688,1601952,892341888,1799786093088,13042490003160192, %T A053400 341378170022783017472,32526326484972756063585792, %U A053400 11367103329997359707194173746176,14669222110846093400698801891700529152 %N A053400 Number of 3-multigraphs on n nodes. %D A053400 F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY,1973. %H A053400 Andrew Howroyd, <a href="/A053400/b053400.txt">Table of n, a(n) for n = 1..50</a> %H A053400 Harald Fripertinger, <a href="http://www.mathe2.uni-bayreuth.de/frib/html/book/hyl00_42.html">The cycle type of the induced action on 2-subsets</a> %H A053400 Vladeta Jovovic, <a href="/A063843/a063843.rtf">Formulae for the number T(n,k) of n-multigraphs on k nodes</a> %o A053400 (Python) %o A053400 from itertools import combinations %o A053400 from math import prod, gcd, factorial %o A053400 from fractions import Fraction %o A053400 from sympy.utilities.iterables import partitions %o A053400 def A053400(n): return int(sum(Fraction(1<<(sum(p[r]*p[s]*gcd(r,s) for r,s in combinations(p.keys(),2))+sum((q>>1)*r+(q*r*(r-1)>>1) for q, r in p.items())<<1),prod(q**r*factorial(r) for q, r in p.items())) for p in partitions(n))) # _Chai Wah Wu_, Jul 09 2024 %Y A053400 Column k=3 of A063841. %Y A053400 Cf. A004102. %K A053400 easy,nonn,nice %O A053400 1,2 %A A053400 _Vladeta Jovovic_, Jan 06 2000