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Every hereditary graph class of bounded order has the Erdős–Hajnal property
Example
Let be a hereditary graph class for which some satisfies for every . Then has the Erdős–Hajnal property.
Facts & Assumptions
Given: A hereditary class and a natural number bounding the order of every member.
The definition applies to hereditary classes and asks for one such that every nonempty satisfies (The Erdős–Hajnal property and an Erdős–Hajnal constant for a hereditary graph class).
The homogeneous number is the larger of the clique and stable-set numbers (Homogeneous vertex sets and the homogeneous number ).
The logarithm is strictly increasing, satisfies , and obeys the quotient law (Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm).
is the inverse function of ; in particular for and for (The natural logarithm as the inverse of the exponential function).
Verification
[assume-case small] If , choose ; every nonempty member has one vertex and homogeneous number by [L2].
[assume-case large] If , choose when , and choose when . In the latter case by [L3], so , and [L4] with [L5] gives .
Since is a strictly increasing bijection onto with inverse , the function is strictly increasing as well; so for the inequality gives by [L4].
In the large case, a graph of order has either an edge, which is a two-vertex clique, or a nonedge, which is a two-vertex stable set; hence by step 2.1, while for both sides equal . The given hereditary hypothesis places in the domain of [L1].
The cases are exhaustive, and in each [L1] supplies the Erdős–Hajnal property.
Depends on
- The Erdős–Hajnal property and an Erdős–Hajnal constant for a hereditary graph class
- Homogeneous vertex sets and the homogeneous number $\operatorname{hom}(G)=\max\{\omega(G),\alpha(G)\}$
- Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm
- Real powers for positive bases, with the zero-base positive-exponent convention
- The natural logarithm as the inverse of the exponential function
Used by
Nothing in the library uses this result yet.
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Sources
- Erdos-Hajnal properties in graphs and hypergraphs, introduction (standard reference, not scraped)