edits

[lambda.git] / week4.mdwn

diff --git a/week4.mdwn b/week4.mdwn
index 10ab8b9..8d89a5a 100644 (file)
--- a/week4.mdwn
+++ b/week4.mdwn
@@ -1,5 +1,7 @@
 [[!toc]]
 
 [[!toc]]
 

+#These notes return to the topic of fixed point combiantors for one more return to the topic of fixed point combinators#
+

 Q: How do you know that every term in the untyped lambda calculus has
 a fixed point?
 
 Q: How do you know that every term in the untyped lambda calculus has
 a fixed point?
 


@@ -7,9 +9,14 @@ A: That's easy: let `T` be an arbitrary term in the lambda calculus.  If
 `T` has a fixed point, then there exists some `X` such that `X <~~>
 TX` (that's what it means to *have* a fixed point).
 
 `T` has a fixed point, then there exists some `X` such that `X <~~>
 TX` (that's what it means to *have* a fixed point).
 

-   let W = \x.T(xx) in
-   let X = WW in
-   X = WW = (\x.T(xx))W = T(WW) = TX
+<pre>
+let W = \x.T(xx) in
+let X = WW in
+X = WW = (\x.T(xx))W = T(WW) = TX
+</pre>
+
+Please slow down and make sure that you understand what justified each
+of the equalities in the last line.

 
 Q: How do you know that for any term T, YT is a fixed point of T?
 
 
 Q: How do you know that for any term T, YT is a fixed point of T?