Abstract
We describe formalization of the Poincaré disc model of hyperbolic geometry within the Isabelle/HOL proof assistant. The model is defined within the complex projective line \(\mathbb {C}{}P^1\)and is shown to satisfy Tarski’s axioms except for Euclid’s axiom—it is shown to satisfy it’s negation, and, moreover, to satisfy the existence of limiting parallels axiom.
Notes
We use the numbering of theorems as of the tenth edition.
To highlight the fact that all of the axioms except for Euclid’s axiom indeed defines a neutral geometry we provide figures both in the Euclidean model and a non-Euclidean model, namely the Poincaré disc model. The figure on the left hand side illustrates the validity of the axiom in Euclidean geometry. The figure on the right hand side either depicts the validity of the statement in the Poincaré disc model or exhibits a counter-example.
From this example it can be seen that the lifting must be done in two stages (one for the subtype and the other for the quotient type). However, to simplify the presentation, in the rest of the paper we shall show only the initial and the final definition.
Note that such claim is slightly imprecise. In a strictly typed setting the value of the cross ratio is a number in the extended complex plane \(\mathbb {C}{}P^1\). If different from \(\infty _h\), then it can be converted to an ordinary complex number and we claim that its imaginary part is equal to 0. Formally we claim that is_real (to_complex (cross_ratio u \(i_1\) v \(i_2\))), where is_real x \(\equiv \) Im x = 0. For simplicity, we shall sometimes make such simplifications.
There already exists tools that translate proofs from one proof assistant to another [1].
there is no development about real closed fields in Isabelle/HOL. This is the reason that motivated us to avoid the restriction of predicates f and g to FOL predicates.
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This work is partially supported by the Serbian Ministry of Education and Science Grant 174021.
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Simić, D., Marić, F. & Boutry, P. Formalization of the Poincaré Disc Model of Hyperbolic Geometry. J Autom Reasoning 65, 31–73 (2021). https://doi.org/10.1007/s10817-020-09551-2
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DOI: https://doi.org/10.1007/s10817-020-09551-2