F22H1 Logic and Proof Week 6 Reasoning

How can we show that this is a tautology (section 11.2): The hard way: “logical calculation” The “easy” way: “reasoning” That’s what chapter 11 is all about: Logical Calculation: use well defined rules one by one and you will slowly get towards the thing that you are trying to prove. Reasoning: (logical calculation, but with a few hints, tips and additional rules) is more like common sense.

Of course, we can do a truth table to show that this is always T – however you can’t do that with, e.g., 100 logical variables. So we need to learn reasoning. We start with this. Why?? Because Lemma Says this: So, if our calculation eventually gets to “P => R”, using steps that are either or then we will have proved what we want to prove

This is the next step: Understand? Next: Tricky! But straightforward

Next this: Understand it? Also known as False-elimination. Next, we can go a long way with rules from chap 7: Actually this only uses AND-weakening

The remainder is fairly quick and easy: So from Lemma We have now proved the tautology we wanted to prove.

Koyaanasqatsi

A brand new dawn … Let’s just assume, for the time being, that the whole first bit is true {Assumption} (1) Now let’s make another assumption: {Assumption} 2)P {from (1) and (2)} 3)Q {from (1) and (3)} 4)R {we assumed P (2) and derived R (4), this means:} 5) P => R {We assumed (1), and got (5), this means:} 6)

Inference This is the general rule for inference: You can be creative about when to make an inference and what to base it on, but of course it has to be valid.

Some valid inference rules {Tautology} (1) {Implication and double negation} (2) {Follows from (1) and (2) – called } (3) {Assume} (1) … {follows from (1) -- (m) {follows from (1) and (m) if there are no assumptions between (1) and (m) that the derivation of (m) relies on -- (m+1)

Reasoning by Contradiction This is an extremely useful inference rule, very often used, often called reductio ad absurdum The idea is: assume the negation of what you want to prove – If reasoning then leads to a contradiction (i.e. leads to False), then you can infer (i.e. you have proved) the negation of your assumption. {Assume} (1) … {via valid inferences but no other assumptions we get …} (m) False {it follows that the original assumption must be False, so we can infer the following} (m+1)

E.g. Let’s prove that 10 is an even number. To do this by Contradiction, we first assume that “10 is even” is False: {Assumption} (1)10 is an odd number {this follows from (1) (2) 10 = 2k + 1 for some k in N {this follows from (2), by simple algebra} (3) 9 = 2k for some k in N {simple algebra, follows from (3)} (4) k = 4.5 and k in N {This follows from (4), since it is a contradiction} (5) False {We have now proved this, via proof by contradiction} (6) 10 is an even number

Prove that this: Implies: It stands to reason, when you look at it. Let’s prove it by contradiction {Assume} (1) { Implication} (2) {De Morgan, double negation, and associativity} (3) {follows from (3) by AND-elimination, twice} (4) {we can see that (4) is a contradiction, so we have proved this: (5)