From 6e55a9652a8a082eceb8efaa0eedbce91a79716f Mon Sep 17 00:00:00 2001 From: Jim Pryor Date: Sun, 19 Sep 2010 22:27:59 -0400 Subject: [PATCH] week2: tweak, undecidability of pred logic Signed-off-by: Jim Pryor --- week2.mdwn | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/week2.mdwn b/week2.mdwn index bd86cbbf..a8e3ca4e 100644 --- a/week2.mdwn +++ b/week2.mdwn @@ -324,13 +324,13 @@ The logical system you get when eta-reduction is added to the proof system has t > if `M`, `N` are normal forms with no free variables, then M ≡ N iff `M` and `N` behave the same with respect to every possible sequence of arguments. -That is, when `M` and `N` are (closed normal forms that are) syntactically distinct, there will always be some sequences of arguments L1, ..., Ln such that: +This implies that, when `M` and `N` are (closed normal forms that are) syntactically distinct, there will always be some sequences of arguments L1, ..., Ln such that: 
M L1 ... Ln x y ~~> x
 N L1 ... Ln x y ~~> y
-That is, closed beta-plus-eta-normal forms will be syntactically different iff they yield different values for some arguments. That is, iff their extensions differ. +So closed beta-plus-eta-normal forms will be syntactically different iff they yield different values for some arguments. That is, iff their extensions differ. So the proof theory with eta-reduction added is called "extensional," because its notion of normal form makes syntactic identity of closed normal forms coincide with extensional equivalence. @@ -432,6 +432,7 @@ But is there any method for doing this in general---for telling, of any given co * [Scooping the Loop Snooper](http://www.cl.cam.ac.uk/teaching/0910/CompTheory/scooping.pdf), a proof of the undecidability of the halting problem in the style of Dr Seuss by Geoffrey K. Pullum +Interestingly, Church also set up an association between the lambda calculus and first-order predicate logic, such that, for arbitrary lambda formulas `M` and `N`, some formula would be provable in predicate logic iff `M` and `N` were convertible. So since the right-hand side is not decidable, questions of provability in first-order predicate logic must not be decidable either. This was the first proof of the undecidability of first-order predicate logic. ##[[Lists and Numbers]]## -- 2.11.0