Mycielskian

In the mathematical area of graph theory, the Mycielskian or Mycielski graph of an undirected graph is a larger graph formed from it by a construction of Jan Mycielski (1955). The construction preserves the property of being triangle-free but increases the chromatic number; by applying the construction repeatedly to a triangle-free starting graph, Mycielski showed that there exist triangle-free graphs with arbitrarily large chromatic number.

Construction

File:Mycielskian construction of 11-vertex Grotzsch graph.png

Mycielskian construction applied to a 5-cycle graph, producing the Grötzsch graph with 11 vertices and 20 edges, the smallest triangle-free 4-chromatic graph .

Let the n vertices of the given graph G be v₁, v₂, . . . , v_n. The Mycielski graph μ(G) contains G itself as a subgraph, together with n+1 additional vertices: a vertex u_i corresponding to each vertex v_i of G, and an extra vertex w. Each vertex u_i is connected by an edge to w, so that these vertices form a subgraph in the form of a star K_1,n. In addition, for each edge v_iv_j of G, the Mycielski graph includes two edges, u_iv_j and v_iu_j.

Thus, if G has n vertices and m edges, μ(G) has 2n+1 vertices and 3m+n edges.

The only new triangles in μ(G) are of the form v_iv_ju_k, where v_iv_jv_k is a triangle in G. Thus, if G is triangle-free, so is μ(G).

To see that the construction increases the chromatic number $\chi (G)=k$, consider a proper k-coloring of $\mu (G)-\{w\}$; that is, a mapping $c:\{v_1,\dots ,v_n,u_1,\dots ,u_n\}\to \{1,2,\dots ,k\}$ with $c(x)\ne c(y)$ for adjacent vertices x,y. If we had $c(u_i)\in \{1,2,\dots ,k-1\}$ for all i, then we could define a proper (k−1)-coloring of G by $c^{\prime }(v_i)=c(u_i)$ when $c(v_i)=k$, and $c^{\prime }(v_i)=c(v_i)$ otherwise. But this is impossible for $\chi (G)=k$, so c must use all k colors for $\{u_1,\dots ,u_n\}$, and any proper coloring of the last vertex w must use an extra color. That is, $\chi (\mu (G))=k+1$.

Iterated Mycielskians

File:Mycielski graphs.svg

M₂, M₃ and M₄ Mycielski graphs

Applying the Mycielskian repeatedly, starting with the one-edge graph, produces a sequence of graphs M_i = μ(M_i−1), sometimes called the Mycielski graphs. The first few graphs in this sequence are the graph M₂ = K₂ with two vertices connected by an edge, the cycle graph M₃ = C₅, and the Grötzsch graph M₄ with 11 vertices and 20 edges.

In general, the graph M_i is triangle-free, (i−1)-vertex-connected, and i-chromatic. The number of vertices in M_i for i ≥ 2 is 3 × 2ⁱ⁻² − 1 (sequence A083329 in the OEIS), while the number of edges for i = 2, 3, . . . is:

1, 5, 20, 71, 236, 755, 2360, 7271, 22196, 67355, ... (sequence A122695 in the OEIS).

Properties

File:Mycielski graph k4 hamiltonian path.svg

Hamiltonian cycle in M₄ (Grötzsch graph)

• If G has chromatic number k, then μ(G) has chromatic number k + 1 .
• If G is triangle-free, then so is μ(G) .
• More generally, if G has clique number ω(G), then μ(G) has clique number of the maximum among 2 and ω(G).
• If G is a factor-critical graph, then so is μ(G) . In particular, every graph M_i for i ≥ 2 is factor-critical.
• If G has a Hamiltonian cycle, then so does μ(G) .
• If G has domination number γ(G), then μ(G) has domination number γ(G)+1 .

Cones over graphs

File:Generalized Mycielskian.svg

A generalized Mycielskian, formed as a cone over the 5-cycle, Δ₃(C₅) = Δ₃(Δ₂(K₂)).

A generalization of the Mycielskian, called a cone over a graph, was introduced by and further studied by and . In this construction, one forms a graph ${\mathrm{\Delta }}_i(G)$ from a given graph G by taking the tensor product G × H, where H is a path of length i with a self-loop at one end, and then collapsing into a single supervertex all of the vertices associated with the vertex of H at the non-loop end of the path. The Mycielskian itself can be formed in this way as μ(G) = Δ₂(G).

While the cone construction does not always increase the chromatic number, proved that it does so when applied iteratively to K₂. That is, define a sequence of families of graphs, called generalized Mycielskians, as

ℳ(2) = {K₂} and ℳ(k+1) = {${\mathrm{\Delta }}_i(G)$ | G ∈ ℳ(k), i ∈ $\mathbb{\mathbb{N}}$}.

For example, ℳ(3) is the family of odd cycles. Then each graph in ℳ(k) is k-chromatic. The proof uses methods of topological combinatorics developed by László Lovász to compute the chromatic number of Kneser graphs. The triangle-free property is then strengthened as follows: if one only applies the cone construction Δ_i for i ≥ r, then the resulting graph has odd girth at least 2r + 1, that is, it contains no odd cycles of length less than 2r + 1. Thus generalized Mycielskians provide a simple construction of graphs with high chromatic number and high odd girth.

References

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