Wikipedia:Reference desk/Archives/Mathematics/2018 September 26

= September 26 =

Mathematics for Computer Science - Propositional Functions
Suppose the domain of the propositional function P(x,y) consists of pairs x and y, where x = 1, 2, or 3, and y = 1, 2, or 3. Write out the following proposition using disjunctions and conjunctions only:

∃xP(x,y)

I don't even know where to start... (x ∧ y) ∨ (y ∧ x) type thing is what the question is asking for? Because I don't see what that would mean anyway, so any help would be greatly appreciated.

Kidoooo$ (talk) 00:21, 26 September 2018 (UTC)
 * I think the kind of expression they have in mind is something like
 * P(1,1) ∨ (P(1,3) ∧ P(2,3))
 * That's not the answer, but it's in the general form the answer would take so hopefully that will get you started.
 * I'm a bit confused though since ∃xP(x,y) has a free variable y, so it defines a propositional function rather than a proposition. Either there is a typo somewhere, or your text/prof is using nonstandard terminology, or the terminology I'm used to is nonstandard (or a combination of these). So maybe the answer is supposed to involve expressions like P(2,y). --RDBury (talk) 05:42, 26 September 2018 (UTC)
 * They may mean for all y there exists an x such that P(x,y). However given just what is here I would assume they meant y to be a free variable. You could do both. Dmcq (talk) 19:47, 26 September 2018 (UTC)


 * Sorry, there actually was a typo. I meant ∃xP(x,3) Kidoooo$ (talk) 22:21, 26 September 2018 (UTC)


 * I also wanted to clarify something about De Morgan's laws. One of them states ¬(p ∧ q) ≡ ¬p ∨ ¬q . Does that also apply to three propositions p, q, and r for example? Like ¬(p ∧ q ∧ r) ≡ ¬p ∨ ¬q ∨ ¬r Kidoooo$ (talk) 23:59, 26 September 2018 (UTC)
 * Well, it's true, and it follows from De Morgan's laws. Literally speaking, it's not one of De Morgan's laws, but it's easily derivable from one (plus transitivity associativity of disjunction). --Trovatore (talk) 00:33, 27 September 2018 (UTC)

I appreciate all of your answers and comments that are helping me understand the material. I also wanted to know if the way I did this is valid or correct:

Suppose the domain of the propositional function P(x, y) consists of pairs x and y, where x = 1, 2, or 3, and y = 1, 2, or 3. Write out the propositions below using disjunctions and conjunctions only.

∃x∀y¬P(x, y)

The above is equivalent to ¬(∀x∃yP(x,y)) So can I write (∀x∃yP(x,y)) using disjunctions and conjunctions and then further adjust it with the negation?

¬(∀x∃yP(x,y)) ≡ ¬((P(1,1) ∨ P(1,2) ∨ P(1,3)) ∧ (P(2,1) ∨ P(2,2) ∨ P(2,3)) ∧ (P(3,1) ∨ P(3,2) ∨ P(3,3))) ≡ ¬(P(1,1) ∨ P(1,2) ∨ P(1,3)) ∨ ¬(P(2,1) ∨ P(2,2) ∨ P(2,3)) ∨ ¬(P(3,1) ∨ P(3,2) ∨ P(3,3)) ≡ (¬P(1,1) ∧ ¬P(1,2) ∧ ¬P(1,3)) ∨ (¬P(2,1) ∧ ¬P(2,2) ∧ ¬P(2,3)) ∨ (¬P(3,1) ∧ ¬P(3,2) ∧ ¬P(3,3))

Is that a valid method and correct answer? Thanks in advance. Kidoooo$ (talk) 02:41, 27 September 2018 (UTC)
 * Yes. I'd have just done it direct but no harm some extra exercise, or perhaps they wanted you to do that. Dmcq (talk) 13:50, 28 September 2018 (UTC)