From 30d676faaf7fd5bd6aff3f2bcae730934c28e6b0 Mon Sep 17 00:00:00 2001 From: tomit4 Date: Sun, 31 May 2026 23:21:59 -0700 Subject: [PATCH] :construction: Started 3.2 --- chapter_3/notes.md | 29 +++++++++++++++++++++++++++++ leftoff.txt | 2 +- 2 files changed, 30 insertions(+), 1 deletion(-) diff --git a/chapter_3/notes.md b/chapter_3/notes.md index b0a4bf6..fb22402 100644 --- a/chapter_3/notes.md +++ b/chapter_3/notes.md @@ -139,3 +139,32 @@ Page 151 - "$\forall x, r(x)$ **only if** $s(x)$" means "$\forall x, \text{ if } \neg s(x) \text{ then } \neg r(x)$" or, equivalently, "$\forall x, \text{ if } r(x) \text{ then } s(x)$." + +--- + +Page 156 + +**Interpreting Statements with Two Different Quantifiers** + +If you want to establish the truth of a statement of the form + +$$ \forall x \text{ in } D, \exists y \text{ in } E \text{ such that } P(x, y) $$ + +your challenge is to allow someone else to pick whatever element $x$ in $D$ they +wish and then you must find an element $y$ in $E$ that "works" for that +particular $x$. + +If you want to establish the truth of a statement of the form + +$$ \exists x \text{ in } D \text{ such that } \forall y \text{ in } E, P(x, y) $$ + +your job is to find one particular $x$ in $D$ that will "work" no matter what +$y$ in $E$ anyone might choose to challenge you with. + +--- + +**Negations of Statements with Two Different Quantifiers** + +$\neg(\forall x \text{ in } d, \exists y \text{ in } E \text{ such that } P(x, y)) \equiv \exists x \text{ in } D \text{ such that } \forall y \text{ in } E, \neg P(x, y)$ + +$\neg(\exists x \text{ in } D \text{ such that } \forall y \text{ in } E, P(x, y)) \equiv \forall x \text{ in } D, \exists y \text{ in } E \text{ such that } \neg P(x, y)$ diff --git a/leftoff.txt b/leftoff.txt index a2ecc45..9e42f3e 100644 --- a/leftoff.txt +++ b/leftoff.txt @@ -1 +1 @@ -154 +165