We show that deterministic collapsible pushdown automata of second order can recognize a language that is not recognizable by any deterministic higher-order pushdown automaton (without collapse) of any order. This implies that there exists a tree generated by a second order collapsible pushdown system (equivalently, by a recursion scheme of second order) that is not generated by any deterministic higher-order pushdown system (without collapse) of any order (equivalently, by any safe recursion scheme of any order). As a side effect, we present a pumping lemma for deterministic higher-order pushdown automata, which potentially can be useful for other applications.

@article{abs-2008-00650,
  title = {On the Expressive Power of Higher-Order Pushdown Systems},
  author = {Paweł Parys},
  journal = {Logical Methods in Computer Science},
  volume = {Volume 16, Issue 3},
  year = {2020},
  doi = {10.23638/lmcs-16(3:11)2020}
}