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A368924 Triangle read by rows where T(n,k) is the number of labeled loop-graphs on n vertices with k loops and n-k non-loops such that it is possible to choose a different vertex from each edge.

%I A368924 #14 Jan 14 2024 16:10:56
%S A368924 1,0,1,0,2,1,1,9,6,1,15,68,48,12,1,222,720,510,150,20,1,3670,9738,
%T A368924 6825,2180,360,30,1,68820,159628,110334,36960,6895,735,42,1,1456875,
%U A368924 3067320,2090760,721560,145530,17976,1344,56,1,34506640,67512798,45422928,15989232,3402756,463680,40908,2268,72,1
%N A368924 Triangle read by rows where T(n,k) is the number of labeled loop-graphs on n vertices with k loops and n-k non-loops such that it is possible to choose a different vertex from each edge.
%C A368924 The axiom of choice says that, given any set of nonempty sets Y, it is possible to choose a set containing an element from each. The strict version requires this set to have the same cardinality as Y, meaning no element is chosen more than once.
%H A368924 Andrew Howroyd, <a href="/A368924/b368924.txt">Table of n, a(n) for n = 0..1325</a> (rows 0..50)
%H A368924 Wikipedia, <a href="https://en.wikipedia.org/wiki/Axiom_of_choice">Axiom of choice</a>.
%F A368924 E.g.f.: A(x,y) = exp(-log(1-T(x))/2 - T(x)/2 + y*T(x) - T(x)^2/4) where T(x) = -LambertW(-x) is the e.g.f. of A000169. - _Andrew Howroyd_, Jan 14 2024
%e A368924 Triangle begins:
%e A368924       1
%e A368924       0      1
%e A368924       0      2      1
%e A368924       1      9      6      1
%e A368924      15     68     48     12      1
%e A368924     222    720    510    150     20      1
%e A368924    3670   9738   6825   2180    360     30      1
%e A368924   68820 159628 110334  36960   6895    735     42      1
%e A368924 Row n = 3 counts the following loop-graphs:
%e A368924   {{1,2},{1,3},{2,3}}  {{1},{1,2},{1,3}}  {{1},{2},{1,3}}  {{1},{2},{3}}
%e A368924                        {{1},{1,2},{2,3}}  {{1},{2},{2,3}}
%e A368924                        {{1},{1,3},{2,3}}  {{1},{3},{1,2}}
%e A368924                        {{2},{1,2},{1,3}}  {{1},{3},{2,3}}
%e A368924                        {{2},{1,2},{2,3}}  {{2},{3},{1,2}}
%e A368924                        {{2},{1,3},{2,3}}  {{2},{3},{1,3}}
%e A368924                        {{3},{1,2},{1,3}}
%e A368924                        {{3},{1,2},{2,3}}
%e A368924                        {{3},{1,3},{2,3}}
%t A368924 Table[Length[Select[Subsets[Subsets[Range[n],{1,2}],{n}], Count[#,{_}]==k&&Length[Select[Tuples[#], UnsameQ@@#&]]!=0&]],{n,0,5},{k,0,n}]
%o A368924 (PARI) T(n)={my(t=-lambertw(-x + O(x*x^n))); [Vecrev(p) | p <- Vec(serlaplace(exp(-log(1-t)/2 - t/2 + t*y - t^2/4)))]}
%o A368924 { my(A=T(8)); for(i=1, #A, print(A[i])) } \\ _Andrew Howroyd_, Jan 14 2024
%Y A368924 Column k = n-1 is A002378.
%Y A368924 The case of a unique choice is A061356, row sums A000272.
%Y A368924 Column k = 0 is A137916, unlabeled version A137917.
%Y A368924 Row sums appear to be A333331.
%Y A368924 The complement has row sums A368596, covering case A368730.
%Y A368924 The unlabeled version is A368926.
%Y A368924 Without the choice condition we have A368928, A116508, A367863, A368597.
%Y A368924 A000085, A100861, A111924 count set partitions into singletons or pairs.
%Y A368924 A006125 counts graphs, unlabeled A000088.
%Y A368924 A006129 counts covering graphs, unlabeled A002494.
%Y A368924 A014068 counts loop-graphs, unlabeled A000666.
%Y A368924 Cf. A000169, A057500, A062740, A129271, A133686, A322661, A367869, A367902, A368601, A368835, A368836, A368927.
%K A368924 nonn,tabl
%O A368924 0,5
%A A368924 _Gus Wiseman_, Jan 10 2024
%E A368924 a(36) onwards from _Andrew Howroyd_, Jan 14 2024