%I #26 Jan 01 2023 15:58:27
%S 1,6,17,36,59,87,114,145,173,205,233,265,293,325,353,385,413,445,473,
%T 505,533,565,593,625,653,685,713,745,773,805,833,865,893,925,953,985,
%U 1013,1045,1073,1105,1133,1165,1193,1225,1253,1285,1313,1345,1373,1405,1433
%N Number of n-regular, N_0-weighted pseudographs on 2 vertices with total edge weight 5, up to isomorphism.
%C Pseudographs are finite graphs with undirected edges without identity, where parallel edges between the same vertices and loops are allowed.
%H Lars Göttgens, <a href="/A358245/b358245.txt">Table of n, a(n) for n = 1..10000</a>
%H J. Flake and V. Mackscheidt, <a href="https://arxiv.org/abs/2206.08226">Interpolating PBW Deformations for the Orthosymplectic Groups</a>, arXiv:2206.08226 [math.RT], 2022.
%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Pseudograph.html">Pseudograph</a>.
%F Apparently a(n) = a(n-1) + a(n-2) - a(n-3) for n >= 11. - _Hugo Pfoertner_, Dec 02 2022
%e For n = 2 the a(2) = 6 such pseudographs are: 1. two vertices connected by a 5-edge and a 0-edge, 2. two vertices connected by a 4-edge and a 1-edge, 3. two vertices connected by a 3-edge and a 2-edges, 4. two vertices where one has a 5-loop and the other one has a 0-loop, 5. two vertices where one has a 4-loop and the other one has a 1-loop, 6. two vertices where one has a 3-loop and the other one has a 2-loop.
%o (Julia)
%o using Combinatorics
%o function A(n::Int)
%o sum_total = 5
%o result = 0
%o for num_loops in 0:div(n, 2)
%o num_cross = n - 2 * num_loops
%o for sum_cross in 0:sum_total
%o for sum_loop1 in 0:sum_total-sum_cross
%o sum_loop2 = sum_total - sum_cross - sum_loop1
%o if sum_loop2 == sum_loop1
%o result +=
%o div(
%o npartitions_with_zero(sum_loop2, num_loops) *
%o (npartitions_with_zero(sum_loop2, num_loops) + 1),
%o 2,
%o ) * npartitions_with_zero(sum_cross, num_cross)
%o elseif sum_loop2 > sum_loop1
%o result +=
%o npartitions_with_zero(sum_loop2, num_loops) *
%o npartitions_with_zero(sum_loop1, num_loops) *
%o npartitions_with_zero(sum_cross, num_cross)
%o end
%o end
%o end
%o end
%o return result
%o end
%o function npartitions_with_zero(n::Int, m::Int)
%o if m == 0
%o if n == 0
%o return 1
%o else
%o return 0
%o end
%o else
%o return Combinatorics.npartitions(n + m, m)
%o end
%o end
%o print([A(n) for n in 1:51])
%Y Other total edge weights: A358243 (3), A358244 (4), A358246 (6), A358247 (7), A358248 (8), A358249 (9).
%K nonn
%O 1,2
%A _Lars Göttgens_, Nov 04 2022