### Abstract

We show that for every sufficiently large n, the number of monotone subsequences of length four in a permutation on n points is at least [EQUATION PRESENTED] Furthermore, we characterize all permutations on [n] that attain this lower bound. The proof uses the flag algebra framework together with some additional stability arguments. This problem is equivalent to some specific type of edge colourings of complete graphs with two colours, where the number of monochromatic K_{4} is minimized. We show that all the extremal colourings must contain monochromatic K_{4} only in one of the two colours. This translates back to permutations, where all the monotone subsequences of length four are all either increasing, or decreasing only.

Original language | English (US) |
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Pages (from-to) | 658-679 |

Number of pages | 22 |

Journal | Combinatorics Probability and Computing |

Volume | 24 |

Issue number | 4 |

DOIs | |

State | Published - Jul 1 2015 |

### ASJC Scopus subject areas

- Theoretical Computer Science
- Statistics and Probability
- Computational Theory and Mathematics
- Applied Mathematics

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## Cite this

*Combinatorics Probability and Computing*,

*24*(4), 658-679. https://doi.org/10.1017/S0963548314000820