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Draft ordered field

let's talk about ordered fields now we've talked about fields so I'm taking a field F and we've actually talked about ordered sets so we'll say F is a field in fact an ordered field if and Here I am assuming that simply the set F is an ordered set so f is just an ordered set but moreover that F is an ordered field if these two things these are the real two conditions if we have three elements here XY and z in F take any three elements well then X less than Y and remember here the less than sign this is our relation this is our ordered relation and it's one that you're probably very familiar with X less than Y well this would imply that X plus Z is less than y plus Z we can add to both sides of an inequality very convenient definitely something you'd like to do and then number two here second second part of the definition of an ordered field if we have two elements x and y in F which are both positive so X is greater than zero and Y is greater than zero well then this implies x times y is greater than zero in other words if I have two positive numbers here or they're not technically numbers and I'm talking about a generic field but you can think of this as how positive times a positive is is a positive right this makes sense and you can really just think about the fact that we're going to be talking about real numbers if I multiply two positive real numbers the result should be positive as well let's let F be an order field and let's take any four elements X Y Z and W in F then we're gonna get these following results and these are things that you would really expect but we're gonna state them just to be perfectly clear number one here if X is positive well then minus X is negative 4 of X is greater than 0 then minus X is less than 0 and we'll say and vice-versa here you know if X is negative then negative x is positive just like you would expect number 2 X positive and y less than Z well then x times y is less than x times Z or so X is positive and Y is less than Z well then multiplying by something less than Z should be less than multiplying by something which is Z number 3 very similar x- + y less than Z this would imply that X Y is greater then X is a very very similar result number 4 X not equal to 0 well that would imply x squared is positive or it's square square a real number it should be positive here this is the way you should be thinking but this is true of an arbitrary ordered field number 5 X positive but less than Y well this implies that 1 over Y is positive but also less than 1 over X think about how you would you know change things around with this inequality think about dividing both sides by x + y not really talking about what that means but this is a true fact number 6 if X is positive but less than Y well then their squares have this relation x squared is less than Y square exactly what you would expect think 1 is less than 2 well 1 squared is less than 2 squared of course and then finally we have a bit of transitivity here notice the less than or equal to sine X less than or equal to Y and Z less than or equal to W well this would imply that X plus Z is less than or equal to y plus W in a sense I can sort of add these inequalities together so we have all of these results these are all true facts if we have an ordered field I'm not going to prove all of them but maybe we'll just prove number one here for you so for number one we assume we assume that X X is in our order field and that X is positive okay so this is my only assumption along with the definition we had of an ordered field now if you remember what was the first definition of the ordered field that for any XY and Z and F with X being less than Y this implies that X plus Z is less than y plus Z this was our first condition of being an ordered field now we also have the fact that well we're working with a field and what were some of the things that happened with the field well we knew that there was a zero in the field so zero is in the field and we also have a minus X in the field these were two conditions of the field field axioms if you want to look back at a previous video on fields you can check those out but let's use all of these facts let's use this that we have we have this and we have this so we assumed X is greater than zero well let's just say that X plus our minus X has to be greater than 0 plus minus X all right these are all field elements and then we'll just use the properties of the field remember X plus minus X one of the field axioms this is in fact zero on the left and any element in the field here minus X plus zero is simply that element so this left hand side I'm using properties of inverses essentially and on the right hand side I'm using properties of the additive identity and hey there we go this is exactly what we wanted to prove draw a little square there if you like some people like that and and that's exactly what we want that if X is greater than 0 it implies that 0 is greater than minus X