Splittability and 1-amalgamability of permutation classes

A permutation class $C$ is splittable if it is contained in a merge of two of its proper subclasses, and it is 1-amalgamable if given two permutations $\sigma$ and $\tau$ in $C$, each with a marked element, we can find a permutation $\pi$ in $C$ containing both $\sigma$ and $\tau$ such that the two marked elements coincide. It was previously shown that unsplittability implies 1-amalgamability. We prove that unsplittability and 1-amalgamability are not equivalent properties of permutation classes by showing that the class $Av(1423, 1342)$ is both splittable and 1-amalgamable. Our construction is based on the concept of LR-inflations, which we introduce here and which may be of independent interest.Comment: 17 pages, 7 figure

Generalized Coloring of Permutations

A permutation pi is a merge of a permutation sigma and a permutation tau, if we can color the elements of pi red and blue so that the red elements have the same relative order as sigma and the blue ones as tau. We consider, for fixed hereditary permutation classes C and D, the complexity of determining whether a given permutation pi is a merge of an element of C with an element of D. We develop general algorithmic approaches for identifying polynomially tractable cases of merge recognition. Our tools include a version of nondeterministic logspace streaming recognizability of permutations, which we introduce, and a concept of bounded width decomposition, inspired by the work of Ahal and Rabinovich. As a consequence of the general results, we can provide nontrivial examples of tractable permutation merges involving commonly studied permutation classes, such as the class of layered permutations, the class of separable permutations, or the class of permutations avoiding a decreasing sequence of a given length. On the negative side, we obtain a general hardness result which implies, for example, that it is NP-complete to recognize the permutations that can be merged from two subpermutations avoiding the pattern 2413

Structural properties of hereditary permutation classes

A permutation class C is splittable if it is contained in a merge of its two proper subclasses, and it is 1-amalgamable if given two permutations σ, τ ∈ C, each with a marked element, we can find a permutation π ∈ C containing both σ and τ such that the two marked elements coincide. In this thesis, we study both 1-amalgamability and splittability of permutation classes. It was previously shown that unsplittability implies 1-amalgamability. We prove that unsplittability and 1-amalgamability are not equivalent properties of permutation classes by showing that there is a permutation class that is both splittable and 1-amalgamable. Moreover, we show that there are infinitely many such classes. Our construction is based on the concept of LR-inflations or more generally on hereditary 2-colorings, which we both introduce here and which may be of independent interest.

Strukturální vlastnosti dědičných tříd permutací

A permutation class C is splittable if it is contained in a merge of its two proper subclasses, and it is 1-amalgamable if given two permutations σ, τ ∈ C, each with a marked element, we can find a permutation π ∈ C containing both σ and τ such that the two marked elements coincide. In this thesis, we study both 1-amalgamability and splittability of permutation classes. It was previously shown that unsplittability implies 1-amalgamability. We prove that unsplittability and 1-amalgamability are not equivalent properties of permutation classes by showing that there is a permutation class that is both splittable and 1-amalgamable. Moreover, we show that there are infinitely many such classes. Our construction is based on the concept of LR-inflations or more generally on hereditary 2-colorings, which we both introduce here and which may be of independent interest. 1Permutační třída C je splittovatelná pokud je obsažena v mergi svých dvou vlastních podtříd, a je 1-amalgamovatelná pokud pro libovolné permutace σ, τ ∈ C, každá s jedním vyznačeným prvkem, dokážeme najít permutaci π ∈ C, která obsahuje σ i τ tak že jejich vyznačené prvky splývají. V této práci zkoumáme 1-amalgamovatelnost a splittovatelnost permutačních tříd. Již dříve bylo dokázáno, že nesplittovatelnost implikuje 1-amalgamovatelnost. My ukážeme, že tyto dvě vlastnosti permutačních tříd nejsou ekvivalentní nalezením permutační třídy, která je splittovatelná a zároveň 1-amalgamovatelná. Navíc ukážeme, že existuje nekonečně mnoho takových permutačních tříd. Naše konstrukce je založená na konceptu LR-nafouknutí nebo více obecně na dědičných 2-obarvení, které v této práci také zavedeme a které by mohly být zajímavé i mimo naše použití. 1Computer Science Institute of Charles UniversityInformatický ústav Univerzity KarlovyFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult