MHB Proving Properties of Lattices: How to Use DeMorgan's Laws

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The discussion focuses on proving properties of bounded, complemented, distributive lattices using DeMorgan's laws. The user has successfully completed the first two proofs and seeks assistance with the third property, which involves demonstrating the equivalence between $x \wedge y \leq z$ and $y \leq x^c \vee z$. They provide a detailed proof attempt, utilizing distributive and commutative laws, and express uncertainty about the reversibility of their steps. Another participant highlights a crucial logical connection that the user had overlooked, aiding their understanding of the problem. The conversation emphasizes collaborative learning and the challenges of self-study in advanced mathematical concepts.
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My problem is this:

Let $L$ be a bounded, complemented, distributive lattice and let $x,y,z\in L$. Prove the following:

1. $x\wedge y = \bot \Leftrightarrow x\leq y^c$
2. $x = (x^c)^c$
3. $x\wedge y \leq z \Leftrightarrow y\leq x^c \vee z$
4. $(x\vee y)^c = x^c \wedge y^c$
5. $(x\wedge y)^c = x^c \vee y^c$

The first two I have finished, and the last two are basically DeMorgan's laws. I'm having some trouble with #3. Any help is appreciated!
 
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Here is one direction of 3:

$y \leq (x^c \vee z) \iff y = y \wedge (x^c \vee z)$

Thus:

$x \wedge y = x \wedge [y \wedge (x^c \vee z)]$

$= x \wedge [(y \wedge x^c) \vee (y \wedge z)]$ (distributive law)

$= [x \wedge (y \wedge x^c) ] \vee [x \wedge (y \wedge z)]$ (distributive law, again)

$ = [x \wedge (x^c \wedge y)] \vee [x \wedge (y \wedge z)]$ (commutativity)

$ = [(x \wedge x^c) \wedge y] \vee [x \wedge (y \wedge z)]$ (associativity)

$ = [0 \wedge y] \vee [x \wedge (y \wedge z)]$ (complement law)

$ = 0 \vee [x \wedge (y \wedge z)]$ (zero law? forget what this is called)

$= x \wedge (y \wedge z)$ (identity law)

$= (x \wedge y) \wedge z$ (associativity)

which shows that $x \wedge y = (x \wedge y) \wedge z$, that is:

$x \wedge y \leq z$

Ask yourself, are these steps reversible?
 
Deveno said:
Here is one direction of 3:

$y \leq (x^c \vee z) \iff y = y \wedge (x^c \vee z)$

Thus:

$x \wedge y = x \wedge [y \wedge (x^c \vee z)]$

$= x \wedge [(y \wedge x^c) \vee (y \wedge z)]$ (distributive law)

$= [x \wedge (y \wedge x^c) ] \vee [x \wedge (y \wedge z)]$ (distributive law, again)

$ = [x \wedge (x^c \wedge y)] \vee [x \wedge (y \wedge z)]$ (commutativity)

$ = [(x \wedge x^c) \wedge y] \vee [x \wedge (y \wedge z)]$ (associativity)

$ = [0 \wedge y] \vee [x \wedge (y \wedge z)]$ (complement law)

$ = 0 \vee [x \wedge (y \wedge z)]$ (zero law? forget what this is called)

$= x \wedge (y \wedge z)$ (identity law)

$= (x \wedge y) \wedge z$ (associativity)

which shows that $x \wedge y = (x \wedge y) \wedge z$, that is:

$x \wedge y \leq z$

Ask yourself, are these steps reversible?

Thanks a lot for your help! I hadn't thought about your first $\iff$. That was what I was missing. I'm learning this on my own with monthly meetings with a professor, so sometimes I don't manage to see simple things. Thank you!
 

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