Substitution and χ-boundedness

A class G of graphs is said to be χ-bounded if there is a function f:N→R such that for all G∈G and all induced subgraphs H of G, χ(H)⩽f(ω(H)). In this paper, we show that if G is a χ-bounded class, then so is the closure of G under any one of the following three operations: substitution, gluing along a clique, and gluing along a bounded number of vertices. Furthermore, if G is χ-bounded by a polynomial (respectively: exponential) function, then the closure of G under substitution is also χ-bounded by some polynomial (respectively: exponential) function. In addition, we show that if G is a χ-bounded class, then the closure of G under the operations of gluing along a clique and gluing along a bounded number of vertices together is also χ-bounded, as is the closure of G under the operations of substitution and gluing along a clique together.