English

A non-hereditary Pollyanna class that is not strongly Pollyanna

Combinatorics 2026-05-15 v1

Abstract

Chudnovsky, Cook, Davies, and Oum introduced the notion of Pollyanna graph classes: a class C\mathcal{C} is Pollyanna if for every χ\chi-bounded class F\mathcal{F}, the intersection CF\mathcal{C} \cap \mathcal{F} is polynomially χ\chi-bounded. They further defined C\mathcal{C} to be strongly Pollyanna if it is kk-strongly Pollyanna for some integer kk, meaning that CF\mathcal{C} \cap \mathcal{F} is polynomially χ\chi-bounded for every kk-good class F\mathcal{F}. They asked whether there are Pollyanna graph classes that are not strongly Pollyanna. In this note we answer this question affirmatively, under the literal interpretation that graph classes are not required to be hereditary. We construct a class C\mathcal{C} that is Pollyanna but, for every k1k \ge 1, is not kk-strongly Pollyanna; in particular C\mathcal{C} is not strongly Pollyanna.

Keywords

Cite

@article{arxiv.2605.14547,
  title  = {A non-hereditary Pollyanna class that is not strongly Pollyanna},
  author = {Hongzhang Chen and Kaiyang Lan},
  journal= {arXiv preprint arXiv:2605.14547},
  year   = {2026}
}

Comments

7 pages. Any comments and suggestions are welcome