On Closed Elementary Cuts in Recursively Saturated Models of Peano Arithmetic. - Bozena Piekart (Notre Dame J. Formal Log., 1993)

Automorphisms of Models of True Arithmetic: Subgroups which Extend to a Maximal Subgroup Uniquely. - Bozena Piekart; Henryk Kotlarski (Math. Log. Q., 1994)

Automorphisms of Countable Recursively Saturated Models of PA: Open Subgroups and Invariant Cuts. - Henryk Kotlarski; Bozena Piekart (Math. Log. Q., 1995)

Automorphisms of Models of True Arithmetic: More on Subgroups which Extend to a Maximal One Uniquely. - Henryk Kotlarski; Bozena Piekart (Math. Log. Q., 2000)

Some variations of the Hardy hierarchy. - Henryk Kotlarski; Bozena Piekart (Math. Log. Q., 2005)

More on lower bounds for partitioning alpha-large sets. - Henryk Kotlarski; Bozena Piekart; Andreas Weiermann (Ann. Pure Appl. Log., 2007)