# 9. Szemerédi's graph regularity lemma IV: induced removal lemma | Summary and Q&A

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May 12, 2020
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9. Szemerédi's graph regularity lemma IV: induced removal lemma

## TL;DR

Induced Removal Lemma is a strengthened version of the Graph Removal Lemma, allowing for the removal of induced subgraphs. It is more difficult to prove due to irregular pairs, but the Strong Regularity Lemma provides a solution. This lemma has various applications, including the Infinite Removal Lemma and Property Testing.

## Key Insights

• ❓ The Induced Removal Lemma is a strengthened version of the Graph Removal Lemma, focusing on removing induced subgraphs.
• 👻 The Strong Regularity Lemma is crucial in proving the Induced Removal Lemma, as it allows for the removal of irregular pairs.
• 🈸 The Induced Removal Lemma has applications in various areas, including the Infinite Removal Lemma and Property Testing.
• 💨 The Property Testing algorithm is an efficient way to determine if a graph has a specific property or is epsilon far from having it.

## Transcript

YUFEI ZHAO: We've been spending the past few lectures discussing Szemeredi's Regularity Lemma. And one of the first applications that we discussed of the Regularity Lemma is the triangle removal Lemma. So today, I want to revisit this topic and show you a strengthening of the Removal Lemma for which new regularity techniques are needed. But first, ... Read More

### Q: How does the Induced Removal Lemma differ from the Graph Removal Lemma?

The Induced Removal Lemma focuses on removing induced subgraphs instead of copies, making it more challenging to prove. It requires the use of the Strong Regularity Lemma to handle irregular pairs.

### Q: What is the Strong Regularity Lemma?

The Strong Regularity Lemma provides a partition of the graph into equitably sized parts that are highly regular. This lemma allows for the removal of irregular pairs and is essential in the proof of the Induced Removal Lemma.

### Q: What applications does the Induced Removal Lemma have?

The Induced Removal Lemma has various applications, including the Infinite Removal Lemma, which deals with infinitely many patterns, and Property Testing, which ensures efficient testing of graph properties.

### Q: How does the Property Testing algorithm work?

The Property Testing algorithm samples a small number of vertices and checks if the graph has a specific property. For hereditary properties, such as being triangle-free or planar, the algorithm can determine if the graph is epsilon far from having the property with high probability.

## Summary & Key Takeaways

• The Induced Removal Lemma is a variant of the Graph Removal Lemma that focuses on induced subgraphs instead of copies.

• The proof of the Induced Removal Lemma requires the use of the Strong Regularity Lemma, which allows for the removal of irregular pairs.

• The Strong Regularity Lemma provides a partition of the graph into equitably sized parts that are highly regular, allowing for the removal of induced subgraphs.

• The Induced Removal Lemma has applications in various areas, including the Infinite Removal Lemma, which deals with infinitely many patterns, and Property Testing, where it ensures efficient testing of graph properties.