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 A368761 #33 May 28 2024 16:36:32
%S A368761 1,2,6,24,128,928,9280,129152,2515200,68780544,2647000064,
%T A368761 143580989440,10988411686912,1187350176604160,181232621966082048,
%U A368761 39089521693818912768,11916533065969825808384,5135497592471003032846336,3128995097443083790244380672,2695613904312277811648715554816
%N A368761 Number of labeled split graphs on n vertices such that {1..k} is independent and {k+1..n} is a clique for some k in {0..n}.
%C A368761 Also the number of sign mappings X:([n] choose 2) -> {+,-} such that for any ordered 3-tuple abc we have X(ab)X(ac)X(bc) not in {++-,+--}.
%F A368761 a(n) = 1 + Sum_{k=1..n-1} (2^k-1)*2^((n-1-k)*k).
%p A368761 seq(1 + add((2^k-1)*2^((n-1-k)*k),k=1..n-1),n=1..20); # Georg Fischer_, May 28 2024
%o A368761 (Python) def f(n): return 1+sum((2**k-1)*2**((n-1-k)*k) for k in range(1,n))
%Y A368761 Cf. A048194.
%K A368761 nonn
%O A368761 1,2
%A A368761 _Robert Lauff_ and _Manfred Scheucher_, Jan 05 2024
%E A368761 a(20), a(21) joined by _Georg Fischer_, May 28 2024