Working in a variant of the intersection type assignment system of Coppo, Dezani-Ciancaglini and Venneri [1981], we prove several facts about sets of terms having a given intersection type. Our main result is that every strongly normalizing term M admits a *uniqueness typing*, which is a pair $(\Gamma,A)$ such that 1) $\Gamma \vdash M : A$ 2) $\Gamma \vdash N : A \Longrightarrow M =_{\beta\eta} N$ We also discuss several presentations of intersection type algebras, and the corresponding choices of type assignment rules. Moreover, we show that the set of closed terms with a given type is uniformly separable, and, if infinite, forms an adequate numeral system. The proof of this fact uses an internal version of the B\"ohm-out technique, adapted to terms of a given intersection type.

@article{paperbot1650,
  title = {On sets of terms having a given intersection type},
  author = {Andrew Polonsky and Richard Statman},
  journal = {Logical Methods in Computer Science},
  volume = {Volume 18, Issue 3},
  year = {2022},
  doi = {10.46298/lmcs-18(3:35)2022}
}