The Mycielskian of a Graph
Let ω(G) and χ(G) be the clique number and the chromatic number of a graph G. Mycielski  presented a construction that for any n creates a graph Mn which is triangle-free (ω(G) = 2) with χ(G) > n. The starting point is the complete graph of two vertices (K 2). M(n+1) is obtained from Mn through the operation μ(G) called the Mycielskian of a graph G.
We first define the operation μ(G) and then show that ω(μ(G)) = ω(G) and χ(μ(G)) = χ(G) + 1. This is done for arbitrary graph G, see also . Then we define the sequence of graphs Mn each of exponential size in n and give their clique and chromatic numbers.