Discrete Mathematics

Transcription

1 Discrete Mathematics Jeremy Siek Spring 2010 Jeremy Siek Discrete Mathematics 1 / 24

2 Outline of Lecture 3 1. Proofs and Isabelle 2. Proof Strategy, Forward and Backwards Reasoning 3. Making Mistakes Jeremy Siek Discrete Mathematics 2 / 24

3 Theorems and Proofs In the context of propositional logic, a theorem is just a tautology. In this course, we ll be writing theorems and their s in the Isabelle/Isar language. Here s the syntax for a theorem in Isabelle/Isar. theorem "P" - step 1 step 2.. step n Each step applies an inference rule to establish the truth of some proposition. Jeremy Siek Discrete Mathematics 3 / 24

4 Inference Rules When applying inference rules, use the keyword have to establish intermediate truths and use the keyword show to conclude the surrounding theorem or sub-. Most inference rules can be categorized as either an introduction or elimination rule. Introduction rules are for creating bigger propositions. Elimination rules are for using propositions. We write L i proves P if there is a preceeding step or assumption in the that is labeled L i and whose proposition is P. Jeremy Siek Discrete Mathematics 4 / 24

5 Introduction Rules And Or (1) Or (2) Implies If L i proves P and L j proves Q, then write from L i L j have L k : "P Q".. If L i proves P, then write from L i have L k : "P Q".. If L i proves Q, then write from L i have L k : "P Q".. have L k : "P Q" assume L i : "P". show "Q" Jeremy Siek Discrete Mathematics 5 / 24

6 Introduction Rules, cont d Not have L k : " P" assume L i : "P".. show "False" Hint: The Appendix of our text Isabelle/HOL A Proof Assistant for Higher-Order Logic lists the logical connectives, such as and, and for each of them gives two ways to input them as ASCI text. If you use Emacs (or XEmacs) to edit your Isabelle files, then the x-symbol package can be used to display the logic connectives in their traditional form. Jeremy Siek Discrete Mathematics 6 / 24

7 Using Assumptions Sometimes the thing you need to prove is already an assumption. In this case your job is really easy! If L i proves P, write from L i have "P". Jeremy Siek Discrete Mathematics 7 / 24

8 Example Proof theorem "p p" - show "p p" assume 1: "p" from 1 show "p". Instead of -, you can apply the introduction rule right away. theorem "p p" assume 1: "p" from 1 show "p". Jeremy Siek Discrete Mathematics 8 / 24

9 Exercise theorem "p (p p)" Jeremy Siek Discrete Mathematics 9 / 24

10 Solution theorem "p (p p)" assume 1: "p" from 1 1 show "p p".. Jeremy Siek Discrete Mathematics 10 / 24

11 Elimination Rules And (1) And (2) Or If L i proves P Q, then write from L i have L k : "P".. If L i proves P Q, then write from L i have L k : "Q".. If L i proves P Q, then write note L i moreover { assume L j : "P". have "R" } moreover { assume L m : "Q". have "R" } ultimately have L k : "R".. Jeremy Siek Discrete Mathematics 11 / 24

12 Elimination Rules, cont d Implies Not False If L i proves P Q and L j proves P, then write from L i L j have L k : "Q".. (This rule is known as modus ponens.) If L i proves P and L j proves P, then write from L i L j have L k : "Q".. If L i proves False, then write from L i have L k : "P".. Jeremy Siek Discrete Mathematics 12 / 24

13 Example Proof theorem "(p q) (p q)" assume 1: "p q" from 1 have 2: "p".. from 2 show "p q".. Jeremy Siek Discrete Mathematics 13 / 24

14 Another Proof theorem "(p q) (p r) (q r) r" assume 1: "(p q) (p r) (q r)" from 1 have 2: "p q".. from 1 have 3: "(p r) (q r)".. from 3 have 4: "p r".. from 3 have 5: "q r".. note 2 moreover { assume 6: "p" from 4 6 have "r".. } moreover { assume 7: "q" from 5 7 have "r".. } ultimately show "r".. Jeremy Siek Discrete Mathematics 14 / 24

15 Exercise theorem "(p q) (q r) (p r)" Jeremy Siek Discrete Mathematics 15 / 24

16 Solution theorem "(p q) (q r) (p r)" assume 1: "(p q) (q r)" from 1 have 2: "p q".. from 1 have 3: "q r".. show "p r" assume 4: "p" from 2 4 have 5: "q".. from 3 5 show "r".. Jeremy Siek Discrete Mathematics 16 / 24

17 Forward and Backwards Reasoning And-Intro (forward) And-Intro (backwards) If L i proves P and L j proves Q, then write from L i L j have L k : "P Q".. have L k : "P Q". show "P" next. show "Q" Jeremy Siek Discrete Mathematics 17 / 24

18 Forward and Backwards Reasoning, cont d Or-Intro (1) (forwards) Or-Intro (1) (backwards) If L i proves P, then write from L i have L k : "P Q".. have L k : "P Q" (rule disji1). show "P" Jeremy Siek Discrete Mathematics 18 / 24

19 Forward and Backwards Reasoning, cont d Or-Intro (2) (forwards) Or-Intro (2) (backwards) If L i proves Q, then write from L i have L k : "P Q".. have L k : "P Q" (rule disji2). show "Q" Jeremy Siek Discrete Mathematics 19 / 24

20 Strategy Let the proposition you re trying to prove guide your. Find the top-most logical connective. Apply the introduction rule, backwards, for that connective. Keep doing that until what you need to prove no longer contains any logical connectives. Then work forwards from your assumptions (using elimination rules) until you ve proved what you need. Assumption Forwards Reasoning Backwards Reasoning Conclusion Assumption Jeremy Siek Discrete Mathematics 20 / 24

21 Making Mistakes To err is human. Isabelle will catch your mistakes. Unfortunately, Isabelle is bad at describing your mistake. Consider the following attempted theorem "p (p p)" - show "p (p p)" assume 1: "p" from 1 show "p p" When Isabelle gets to from 1 show "p p" (adding.. at the end), it gives the following response: Failed to finish At command "..". Jeremy Siek Discrete Mathematics 21 / 24

22 Making Mistakes, cont d In this case, the mistake was a missing label in the from clause. Conjuction introduction requires two premises, not one. Here s the fix: theorem "p (p p)" - show "p (p p)" assume 1: "p" from 1 1 show "p p".. When Isablle says no, double check the inference rule. If that doesn t work, get a classmate to look at it. If that doesn t work, the instructor with the minimal Isabelle file that exhibits your problem. Jeremy Siek Discrete Mathematics 22 / 24

23 Making Mistakes, cont d Here s another with a typo: theorem "p p" assume 1: "p" from 1 show "q". Isabelle responds with: Local statement will fail to refine any pending goal Failed attempt to solve goal by exported rule : (p) = q At command " show ". The problem here is that the proposition in the show "q", does not match what we are trying to prove, which is p. Jeremy Siek Discrete Mathematics 23 / 24

24 Stuff to Rememeber How to write Isabelle/Isar s of tautologies in Propositional Logic. The introduction and elimination rules. Forwards and backwards reasoning. Jeremy Siek Discrete Mathematics 24 / 24