OFFSET
1,2
COMMENTS
Pseudographs are finite graphs with undirected edges without identity, where parallel edges between the same vertices and loops are allowed.
LINKS
Lars Göttgens, Table of n, a(n) for n = 1..10000
J. Flake and V. Mackscheidt, Interpolating PBW Deformations for the Orthosymplectic Groups, arXiv:2206.08226 [math.RT], 2022.
Eric Weisstein's World of Mathematics, Pseudograph.
FORMULA
Apparently a(n) = a(n-1) + a(n-2) - a(n-3) for n >= 11. - Hugo Pfoertner, Dec 02 2022
EXAMPLE
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.
PROG
(Julia)
using Combinatorics
function A(n::Int)
sum_total = 5
result = 0
for num_loops in 0:div(n, 2)
num_cross = n - 2 * num_loops
for sum_cross in 0:sum_total
for sum_loop1 in 0:sum_total-sum_cross
sum_loop2 = sum_total - sum_cross - sum_loop1
if sum_loop2 == sum_loop1
result +=
div(
npartitions_with_zero(sum_loop2, num_loops) *
(npartitions_with_zero(sum_loop2, num_loops) + 1),
2,
) * npartitions_with_zero(sum_cross, num_cross)
elseif sum_loop2 > sum_loop1
result +=
npartitions_with_zero(sum_loop2, num_loops) *
npartitions_with_zero(sum_loop1, num_loops) *
npartitions_with_zero(sum_cross, num_cross)
end
end
end
end
return result
end
function npartitions_with_zero(n::Int, m::Int)
if m == 0
if n == 0
return 1
else
return 0
end
else
return Combinatorics.npartitions(n + m, m)
end
end
print([A(n) for n in 1:51])
CROSSREFS
KEYWORD
nonn
AUTHOR
Lars Göttgens, Nov 04 2022
STATUS
approved