F o r m a l i z a t i o n   i n   A g d a   o f   
   v a r i o u s   n e w   t h e o r e m s  i n  
   c o n s t r u c t i v e   m a t h e m a t i c s.


   Martin Escardo, 2012, version of 22 Feb 2012.


\begin{code}

module index where

\end{code}

Some people formalize proofs to get reassurance. Here I instead
formalize my proofs so that I can run them: proofs really are
programs, in a very concrete sense. I unavoidably get reassurance
too, but this is not what motivated me to write the formal
versions of the proofs. 

You can navigate this set of files by clicking at words or
symbols to get to their definitions.


The following module investigates the notion of omniscience set. A
set X is omniscient iff

   ∀(p : X → ₂) → (∃ \(x : X) → p x ≡ ₀) ∨ (∀(x : X) → p x ≡ ₁)

\begin{code}

open import Omniscience 

\end{code}

The omniscience of ℕ is a taboo, known as LPO. See also:

\begin{code}

open import WLPO

\end{code}

An example of an omniscient set is ℕ∞, which intuitively is 
ℕ ∪ { ∞ }, defined in the following module:

\begin{code}

open import GenericConvergentSequence 

\end{code}

But it is more direct to show that ℕ∞ is searchable, and get
omniscience as a corollary:

\begin{code}

open import Searchable 
open import ConvergentSequenceSearchable 

\end{code}

An interesting consequence of the omniscience of ℕ∞ is that the
following property, an instance of WLPO, holds constructively:

  ∀(p : ℕ∞ → ₂) → (∀(n : ℕ) → p(under n) ≡ ₁) 
               ∨ ¬(∀(n : ℕ) → p(under n) ≡ ₁).

\begin{code}

open import ADecidableQuantificationOverTheNaturals

\end{code}

As a side remark, the following module characterizes ℕ∞ as the
final coalgebra of the functor 1+(-), and is followed by an
illustrative example:

\begin{code}

open import CoNaturals 
open import CoNaturalsExercise

\end{code}

The following module discusses in what sense ℕ∞ is the generic
convergent sequence, and proves that the universe Set is
indiscrete, with a certain Rice's Theorem for the universe Set as
a corollary:

\begin{code}

open import TheTopologyOfTheUniverse 
open import RicesTheoremForTheUniverse 

\end{code}

The following two rogue modules depart from our main philosophy of
working strictly within ML type theory with the propositional
axiom of extensionality. They disable the termination checker, for
the reasons explained in the first module. But to make our point,
we also include runnable experiments in the second module:

\begin{code}

open import CountableTychonoff 
open import CantorSearchable 

\end{code}

The following modules return to the well-behavedness paradigm.
The first one shows that a basic form of discontinuity is a
taboo. This, in fact, is used to formulate and prove Rice's
Theorem mentioned above:

\begin{code}

open import BasicDiscontinuityTaboo

\end{code}

The following modules contain auxiliary definitions and additional
results and discussion that we choose not to discuss here:

\begin{code}

open import Cantor 
open import CurryHoward 
open import DecidableAndDetachable 
open import DiscreteAndSeparated 
open import Equality 
open import Exhaustible 
open import Extensionality 
open import FailureOfTotalSeparatedness 
open import FirstProjectionInjective 
open import Image 
open import Injection 
open import Isomorphism 
open import Naturals 
open import Sequence 
open import SetsAndFunctions 
open import Singleton 
open import Singular 
open import Surjection 
open import Two 
open import UIP 

\end{code}