Extension properties for partial permutations

Abstract

Motivated by the EPPA problem for finite tournaments,we consider various extension properties for partial permutations. We show that for any set $Π$ of prime numbers, the $Π$-extension property is equivalent to the $Π$-LERF. As a consequence of known results, the odd-extension property is then equivalent to the EPPA for finite tournaments. To study the $Π$-extension property, we reformate the property using the concepts of EP problems and $Π$-solutions. We then show that the existence of a $Π$-solution for an EP problem depends entirely on its fundamental group. In particular, when the fundamental group is trivial or cyclic, the EP problem has a $Π$-solution for any $Π$. These extend some known results of Huang, Pawliuk, Sabok and Wise [HPSW19]. We give examples of EP problems without nilpotent-solutions; they witness that the $p$-extension property and the $p$-LERF fail for any prime $p$. Then we consider a special kind of EP problems whose fundamental groups have two generators. We show that any 1-weakly wandering problem has a $Π$-solution for any $Π$. For 2-weakly wandering problems, we show that they all have odd-solutions and we completely characterize those without nilpotent-solutions.