cp's OEIS Frontend

A358246 Number of n-regular, N_0-weighted pseudographs on 2 vertices with total edge weight 6, up to isomorphism.

%I A358246 #19 Jan 01 2023 15:59:20
%S A358246 1,8,23,55,92,147,196,260,313,380,434,502,556,624,678,746,800,868,922,
%T A358246 990,1044,1112,1166,1234,1288,1356,1410,1478,1532,1600,1654,1722,1776,
%U A358246 1844,1898,1966,2020,2088,2142,2210,2264,2332,2386,2454,2508,2576,2630,2698
%N A358246 Number of n-regular, N_0-weighted pseudographs on 2 vertices with total edge weight 6, up to isomorphism.
%C A358246 Pseudographs are finite graphs with undirected edges without identity, where parallel edges between the same vertices and loops are allowed.
%H A358246 Lars Göttgens, <a href="/A358246/b358246.txt">Table of n, a(n) for n = 1..10000</a>
%H A358246 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 A358246 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Pseudograph.html">Pseudograph</a>.
%F A358246 Apparently a(n) = a(n-1) + a(n-2) - a(n-3) for n >= 13. - _Hugo Pfoertner_, Dec 02 2022
%e A358246 For n = 2 the a(2) = 8 such pseudographs are: 1. two vertices connected by a 6-edge and a 0-edge, 2. two vertices connected by a 5-edge and a 1-edge, 3. two vertices connected by a 4-edge and a 2-edge, 4. two vertices connected by two 3-edges, 5. two vertices where one has a 6-loop and the other one has a 0-loop, 6. two vertices where one has a 5-loop and the other one has a 1-loop, 7. two vertices where one has a 4-loop and the other one has a 2-loop, 8. two vertices with a 3-loop each.
%o A358246 (Julia)
%o A358246 using Combinatorics
%o A358246 function A(n::Int)
%o A358246     sum_total = 6
%o A358246     result = 0
%o A358246     for num_loops in 0:div(n, 2)
%o A358246         num_cross = n - 2 * num_loops
%o A358246         for sum_cross in 0:sum_total
%o A358246             for sum_loop1 in 0:sum_total-sum_cross
%o A358246                 sum_loop2 = sum_total - sum_cross - sum_loop1
%o A358246                 if sum_loop2 == sum_loop1
%o A358246                     result +=
%o A358246                         div(
%o A358246                             npartitions_with_zero(sum_loop2, num_loops) *
%o A358246                             (npartitions_with_zero(sum_loop2, num_loops) + 1),
%o A358246                             2,
%o A358246                         ) * npartitions_with_zero(sum_cross, num_cross)
%o A358246                 elseif sum_loop2 > sum_loop1
%o A358246                     result +=
%o A358246                         npartitions_with_zero(sum_loop2, num_loops) *
%o A358246                         npartitions_with_zero(sum_loop1, num_loops) *
%o A358246                         npartitions_with_zero(sum_cross, num_cross)
%o A358246                 end
%o A358246             end
%o A358246         end
%o A358246     end
%o A358246     return result
%o A358246 end
%o A358246 function npartitions_with_zero(n::Int, m::Int)
%o A358246     if m == 0
%o A358246         if n == 0
%o A358246             return 1
%o A358246         else
%o A358246             return 0
%o A358246         end
%o A358246     else
%o A358246         return Combinatorics.npartitions(n + m, m)
%o A358246     end
%o A358246 end
%o A358246 print([A(n) for n in 1:48])
%Y A358246 Other total edge weights: A358243 (3), A358244 (4), A358245 (5), A358247 (7), A358248 (8), A358249 (9).
%K A358246 nonn
%O A358246 1,2
%A A358246 _Lars Göttgens_, Nov 04 2022