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Add mycielski
operator
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@@ -851,3 +851,76 @@ function merge_vertices!(g::Graph{T}, vs::Vector{U} where U <: Integer) where T | |||||
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return new_vertex_ids | ||||||
end | ||||||
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""" | ||||||
mycielski(g) | ||||||
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Returns a graph after applying the Mycielski operator to the input. The Mycielski operator | ||||||
returns a new graph with `2n+1` vertices and `3e+n` edges and will increase the chromatic | ||||||
number of the graph by 1. | ||||||
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The Mycielski operation can be repeated by using the `iterations` kwarg. Each time, it will | ||||||
apply the operator to the previous iterations graph. | ||||||
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For each vertex in the original graph, that vertex and a copy of it is added to the new graph. | ||||||
Then, for each edge `(x, y)` in the original graph, the edges `(x, y)`, `(x', y)`, and `(x, y')` | ||||||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I think, the usual convention for variables that represent vertices are the symbols |
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are added to the new graph, where `x'` and `y'` are the "copies" of `x` and `y`, respectively. | ||||||
In otherwords, the original graph is present as a subgraph, and each vertex in the original graph | ||||||
is connected to all of it's neighbors' copies. Finally, one last vertex `w` is added to the graph | ||||||
and an edge connecting each copy vertex `x'` to `w` is added. | ||||||
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See [Mycielskian](https://en.wikipedia.org/wiki/Mycielskian) for more information. | ||||||
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# Examples | ||||||
```jldoctest | ||||||
julia> c = CycleGraph(5) | ||||||
{5, 5} undirected simple Int64 graph | ||||||
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julia> m = Graphs.mycielski(c) | ||||||
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Suggested change
We usually use |
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{11, 20} undirected simple Int64 graph | ||||||
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julia> collect(edges(m)) | ||||||
20-element Vector{Graphs.SimpleGraphs.SimpleEdge{Int64}}: | ||||||
Edge 1 => 2 | ||||||
Edge 1 => 5 | ||||||
Edge 1 => 7 | ||||||
Edge 1 => 10 | ||||||
Edge 2 => 3 | ||||||
Edge 2 => 6 | ||||||
Edge 2 => 8 | ||||||
Edge 3 => 4 | ||||||
Edge 3 => 7 | ||||||
Edge 3 => 9 | ||||||
Edge 4 => 5 | ||||||
Edge 4 => 8 | ||||||
Edge 4 => 10 | ||||||
Edge 5 => 6 | ||||||
Edge 5 => 9 | ||||||
Edge 6 => 11 | ||||||
Edge 7 => 11 | ||||||
Edge 8 => 11 | ||||||
Edge 9 => 11 | ||||||
Edge 10 => 11 | ||||||
``` | ||||||
""" | ||||||
@traitfn function mycielski(g::AbstractGraph::(!IsDirected); iterations = 1) | ||||||
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Suggested change
The problem we have at the moment, that for graphs where we have some kind of edge weights/data, it is not clear what we mean by adding an edge. This is definitely something we should sort out in the future, but in the meantime I would suggest to restrict this function to Also, would suggest to restrict the type of iterations to to |
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ref = g | ||||||
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out = deepcopy(g) | ||||||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I think that There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. These operators are not supposed to mutate the input right? I see some have There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Yes I agree with you. I was only thinking about the case in which we can mutate the input. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I would also support having both |
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for _ in 1:iterations | ||||||
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Suggested change
To be honest though, I am not sure if that makes a big difference for the case where |
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N = nv(g) | ||||||
add_vertices!(out, N + 1) | ||||||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. This might fail, if the graph For the non-mutating |
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w = nv(out) | ||||||
for e in collect(edges(g)) | ||||||
x, y = Tuple(e) | ||||||
add_edge!(out, x, y) | ||||||
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add_edge!(out, x, y+N) | ||||||
add_edge!(out, x+N, y) | ||||||
end | ||||||
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for v in 1:N | ||||||
add_edge!(out, v+N, w) | ||||||
end | ||||||
g = out | ||||||
end | ||||||
return out | ||||||
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end |
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@@ -320,4 +320,20 @@ | |
@testset "Length: $g" for g in testgraphs(SimpleGraph(100)) | ||
@test length(g) == 10000 | ||
end | ||
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@testset "Mycielski Operator" begin | ||
g = complete_graph(2) | ||
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m = mycielski(g; iterations = 8) | ||
@test nv(m) == 767 | ||
@test ne(m) == 22196 | ||
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# check that mycielski preserves triangle-freeness | ||
g = complete_bipartite_graph(10, 5) | ||
m = mycielski(g) | ||
@test nv(m) == 2*15 + 1 | ||
@test ne(m) == 3*50 + 15 | ||
@test all(iszero, triangles(m)) | ||
end | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. These tests are good, but I think we need to cover some additional test cases such as:
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end |
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I think using the symbol
e
for the number of edges is a bit unusual, when we usen
for the number of vertices at the same time. Maybe there is a better way?