一句话:仅用 Σ、Id、Π 与带大消去的二元素类型,从整数类型构造出自然数类型。
在同伦类型论(HoTT)中,自然数类型可视为由一元素与一自同态自由生成,整数类型则由一元素与一自同构自由生成。本文的核心是从整数类型出发构造自然数类型,且只使用依赖和类型、同一类型、外延依赖积,以及带大消去的二元素类型。由此得到推论:仅含 Σ、Id、Π 与带下降(descent)的有限余极限、不含宇宙的同伦类型论也允许自然数类型。作者指出该构造改进并简化了 Rose 的已有结果。
In homotopy type theory, a natural number type is freely generated by an element and an endomorphism. Similarly, an integer type is freely generated by an element and an automorphism. Using only dependent sums, identity types, extensional dependent products, and a type of two elements with large elimination, we construct a natural number type from an integer type. As a corollary, homotopy type theory with only Σ, Id, Π, and finite colimits with descent (and no universes) admits a natural number type. This improves and simplifies a result by Rose.