If anyone would be able to check my answers, and/or point me in the right direction on those which are wrong or I'm stuck on, I'd appreciate it.
Claim 1: One such counterexample would be if we let which is a strictly increasing sequence that is bounded above at the value of 1.
Claim 2: is a counterexample
Claim 3: Stuck on this as I wasn't sure if must be a constant or not, but I settled on the decision that it cannot be. So if then as it would tend to 0 instead, I think.

I suppose this one leads off the previous part. I think this is not necessarily true but I'm unsure how to prove it, and I haven't made much progress here.

RDKGames
 Follow
 98 followers
 3 badges
 Community Assistant
 Welcome Squad
 Thread Starter
Offline3ReputationRep: Follow
 1
 12102016 16:31

 Follow
 2
 12102016 16:57
(Original post by RDKGames)
If anyone would be able to check my answers, and/or point me in the right direction on those which are wrong or I'm stuck on, I'd appreciate it.
Claim 1: One such counterexample would be if we let which is a strictly increasing sequence that is bounded above at the value of 1.
Claim 2: is a counterexample
Claim 3: Stuck on this as I wasn't sure if must be a constant or not, but I settled on the decision that it cannot be. So if then as it would tend to 0 instead, I think.

I suppose this one leads off the previous part. I think this is not necessarily true but I'm unsure how to prove it, and I haven't made much progress here.
If , for example, then your sequence for would still have
For the last bit, rather than look for a counterexample, try and prove it's true.Last edited by ghostwalker; 12102016 at 16:58. 
RDKGames
 Follow
 98 followers
 3 badges
 Community Assistant
 Welcome Squad
 Thread Starter
Offline3ReputationRep: Follow
 3
 12102016 18:58
(Original post by ghostwalker)
Your counterexample for claim 3 is incomplete. You need to specify an sequence to go with that
If , for example, then your sequence for would still have
For the last bit, rather than look for a counterexample, try and prove it's true.
Since we can say that
Then we know that so we can say it is
Leaving us with which indeed tends to infinity.
Generalising exponents we can also get which is always tending to infinity by the looks of it, but I do not feel like this is a good enough proof as it doesn't seem to branch out to different functions tending to infinity.Last edited by RDKGames; 12102016 at 19:01. 
 Follow
 4
 12102016 19:42
(Original post by RDKGames)
I suppose this one leads off the previous part. I think this is not necessarily true but I'm unsure how to prove it, and I haven't made much progress here.

 Follow
 5
 13102016 08:37
(Note: A sequence isn't necessarily going to have a nice formula defining it.)
We need to start with, what does it mean a sequence tends to infinity (which is what we've been given in the question for a_n)? What's the mathematical definition you've been given?
Qualitatively, we can see that if a sequence goes off to infinity, and we multiply each element of it by at least half, it's still going to go off to infinity, only it's rate of growth may be slower.
PS: Analysis proofs tend to be a shock to the system when first encountered. Can be a bit like you're sailing along nicely, then suddenly lolwtf!!Last edited by ghostwalker; 13102016 at 10:27. 
RDKGames
 Follow
 98 followers
 3 badges
 Community Assistant
 Welcome Squad
 Thread Starter
Offline3ReputationRep: Follow
 6
 13102016 13:32
(Original post by ghostwalker)
Yes.
Well that's a specific example of two sequences that meet the criterion.
Whilst a specific example can sometimes help to give you the idea of how to go about a general proof, I think it will take longer to analyse how/why that's happening here and we could get distracted by details in the specific sequences, than it would be to do it for the general case.
(Note: A sequence isn't necessarily going to have a nice formula defining it.)
We need to start with, what does it mean a sequence tends to infinity (which is what we've been given in the question for a_n)? What's the mathematical definition you've been given?
Qualitatively, we can see that if a sequence goes off to infinity, and we multiply each element of it by at least half, it's still going to go off to infinity, only it's rate of growth may be slower.
PS: Analysis proofs tend to be a shock to the system when first encountered. Can be a bit like you're sailing along nicely, then suddenly lolwtf!!
if, and only if, such that
and this makes sense to me but I am struggling to apply it when introducing, within a proof, a different sequence that has to be strictly above .
Since and if then which still tends to infinity 
 Follow
 7
 13102016 14:59
(Original post by RDKGames)
The definition for a sequence tending to infinity has been given as:
if, and only if, such that
has no meaning as such.
We know that for any value C>0 we choose, we can find N (dependent on C) such that
We want to show (I use different letters to avoid confusion),
for any value D>0 we choose, we can find M (dependent on D) such that
The problem becomes tying these two definitions together.
Edit: Sorry, struggling to explain clearly and have to go out just now. Will get back to it again in a few hours. 
 Follow
 8
 13102016 15:45
(Original post by RDKGames)
The definition for a sequence tending to infinity has been given as:
if, and only if, such that
and this makes sense to me but I am struggling to apply it when introducing, within a proof, a different sequence that has to be strictly above .
Although we usually interpret as "for all", or "for every", it's often more helpful to think of it as "for any".
Similarly, rather than thinking of as "exists", think of it as "we can find".
So your statement becomes " if for any C > 0, we can find such that for any "
Moving on, with a similar rephrasing, the result we actually want to prove (that ) is:
"for any C > 0, we can find such that for any "
In a way this is just playing with words, but hopefully it makes it a bit easier to understand that to prove the statement above, your answer is going to look like:
Take C > 0; here's how we can find such that for any ,
(where the bit I've underlined is where the "real work" of the proof is going to lie).
To do the underlined bit, you're going to need to use the definiton for in order to find a suitable "N".
Edit: regarding going from "for any C" to "Take C > 0". It basically just saves a bit of writing/mental effort. Rather than continually doing/thinking "here's how you do it for all values of C", it's easier to write/think "here's how you do it for a particular value of C" (this is particularly true when you want to use C as a value in another definition). But since the "particular value" was arbitrary, at the end you've still shown it for arbitary C.Last edited by DFranklin; 13102016 at 15:51. 
 Follow
 9
 13102016 20:39
(Original post by DFranklin)
Hope ghostwalker won't mind me taking a stab at clarifying.
(Original post by RDKGames)
...
With C,D,M,N symbols I defined in my previous post.
Showing is unbounded:
We have an arbitrary D > 0.
By the unboundedness of and choosing C equal to D
But we're interested in , and since we can say
and since
We now choose our M to equal N.
So, we've shown:
Which is almost what we want, but not quite. There's a pesky "/2". So, we need to adjust this, somewhere in the point we either go into, or come out of our orginal sequence 
 Follow
 10
 13102016 22:28
(Original post by ghostwalker)
Which is almost what we want, but not quite. There's a pesky "/2". So, we need to adjust this, somewhere in the point we either go into, or come out of our orginal sequence
Doesn't D being an arbitrary value not just imply the sequence tends to infinity anyway? 
 Follow
 11
 14102016 08:21
(Original post by B_9710)
Doesn't D being an arbitrary value not just imply the sequence tends to infinity anyway?
BUT, it's more important that the OP gets a feel for how these proofs are done (even though I don't feel I've explained it very well) and their exactness, than look for the "shortcuts" (for want of a better term). 
RDKGames
 Follow
 98 followers
 3 badges
 Community Assistant
 Welcome Squad
 Thread Starter
Offline3ReputationRep: Follow
 12
 14102016 13:53
(Original post by ghostwalker)
Not in the slightest, and much appreciated. I have added notation/definitions for the sake of clarity of the explanation that wouldn't be part of a standard proof, but I can't see offhand how to avoid this. Please feel free to criticise  think I'm making a bit of a pig's ear of it.
OK, lets take a stab at a proof. Whilst the format below should be sound, it won't actually work, but hopefully you can see how it could be adapted.
With C,D,M,N symbols I defined in my previous post.
Showing is unbounded:
We have an arbitrary D > 0.
By the unboundedness of and choosing C equal to D
But we're interested in , and since we can say
and since
We now choose our M to equal N.
So, we've shown:
Which is almost what we want, but not quite. There's a pesky "/2". So, we need to adjust this, somewhere in the point we either go into, or come out of our orginal sequence
Would this be the correct approach?
Analysis seems like out of this world so far as I am 2 weeks into it aha, trying to get used to all of it is proving to be a hurdle! 
 Follow
 13
 14102016 15:33
(Original post by RDKGames)
Ah that is a good example of how to approach these proofs, thank you! So for the D/2 at the end, would you just multiply both sides by 2 and show that ?? Other method of getting rid off that "/2", as you say, I think would be to let a different variable equal D/2.
Would this be the correct approach?
Analysis seems like out of this world so far as I am 2 weeks into it aha, trying to get used to all of it is proving to be a hurdle!
Saying seems and is obvious, but prove it from the definitions. There are a several simple results that it is useful to use/have, but you really need to prove them first before using them.]
"let a different variable equal D/2." may be heading in the right direction, but I'm not clear what you mean.
I think it best to just say:
We chose D to start with, and then we chose C based on D, but there is no requirement for them to be equal. So, suppose we let C=2D.... 
RDKGames
 Follow
 98 followers
 3 badges
 Community Assistant
 Welcome Squad
 Thread Starter
Offline3ReputationRep: Follow
 14
 16102016 11:28
(Original post by ghostwalker)
[I should emphaszie I'm being pedantic here.
Saying seems and is obvious, but prove it from the definitions. There are a several simple results that it is useful to use/have, but you really need to prove them first before using them.]
"let a different variable equal D/2." may be heading in the right direction, but I'm not clear what you mean.
I think it best to just say:
We chose D to start with, and then we chose C based on D, but there is no requirement for them to be equal. So, suppose we let C=2D.... 
RDKGames
 Follow
 98 followers
 3 badges
 Community Assistant
 Welcome Squad
 Thread Starter
Offline3ReputationRep: Follow
 15
 19102016 14:50
ghostwalker DFranklin
Practicing the ways of Analysis proofs I stumbled on this one though it seems as if it should be straightforward. I'm unsure;
(a) if, and only if,
(b) We know that if then
We want the conclusion that
So if and if
Since
but this doesn't make sense as is negative and is positive, so the negative cannot be greater than the positive. Not sure if taking the modulus of the RHS would resolve this. 
 Follow
 16
 19102016 15:08
(Original post by RDKGames)
,,
To give a concrete example of why it's confusing, look at your line
I don't know if you mean "if epsilon > 0 then 1/epsilon > 0 and ...", or you mean "if it is true that 'epsilon > 0 implies 1/epsilon > 0' and ..."
It is usually a lot clearer to simply write "so" or "then" if you are just using normal mathematical arguments.
In this case I am guessing you meant something along the lines of
"So, if epsilon > 0 then 1/epsilon > 0, and if 0 > C then 1/epsilon > C".
As far as the actual proof here, I would focus on why (by intuition) you would expect the result to be true.
That is: if a_n is consistently large and negative, then 1/a_n is consistently small and negative, so 1/a_n is small and positive.
You want to show 1/a_n < epsilon. Working backwards shows you you will need 0 > 1/a_n > epsilon (note use of > since a_n and  epsilon are both negative). So you need a_n < 1/epsilon. 
RDKGames
 Follow
 98 followers
 3 badges
 Community Assistant
 Welcome Squad
 Thread Starter
Offline3ReputationRep: Follow
 17
 19102016 18:19
(Original post by DFranklin)
I don't know if you mean "if epsilon > 0 then 1/epsilon > 0 and ...", or you mean "if it is true that 'epsilon > 0 implies 1/epsilon > 0' and ..."
Here I simply worked from our wanted form that comes straight from the definition: , and so this tells us that it must be true that before proceeding on with the argument.
As far as the actual proof here, I would focus on why (by intuition) you would expect the result to be true.
That is: if a_n is consistently large and negative, then 1/a_n is consistently small and negative, so 1/a_n is small and positive.
You want to show 1/a_n < epsilon. Working backwards shows you you will need 0 > 1/a_n > epsilon (note use of > since a_n and  epsilon are both negative). So you need a_n < 1/epsilon.
and but how do you get the inequality you've shown from this? 
 Follow
 18
 19102016 18:26
if b_{n} = 1/a_{n} that would be a counter example for c)

RDKGames
 Follow
 98 followers
 3 badges
 Community Assistant
 Welcome Squad
 Thread Starter
Offline3ReputationRep: Follow
 19
 19102016 18:31
(Original post by the bear)
if b_{n} = 1/a_{n} that would be a counter example for c)
Write a reply…
Reply
Submit reply
Updated: October 19, 2016
Share this discussion:
Tweet
Related discussions:
 Circuit Analysis Questions.
 Difficulty deciding amongst Maths PhD offers!
 Calling all University Mathematicians!
 Mathematical Economical Problems
 Anyone currently studying Mathematics at Imperial/Warwick/St ...
 Best thing to do over the summer for uni preparation
 Discrete Mathematics Fresher Advice
 Warwick MORSE
 Any fellow PhD Mathematics students
 Preparing for University Maths.
TSR Support Team
We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.
This forum is supported by:
 SherlockHolmes
 Notnek
 charco
 Mr M
 TSR Moderator
 Nirgilis
 usycool1
 Changing Skies
 James A
 rayquaza17
 RDKGames
 randdom
 davros
 Gingerbread101
 Kvothe the Arcane
 Airmed
 The Financier
 The Empire Odyssey
 Protostar
 surina16
 nisha.sri
 Reality Check
 claireestelle
 Doonesbury
 furryface12
 Amefish
 harryleavey
 Lemur14
 brainzistheword
 Rexar
 Sonechka
 LeCroissant
 EstelOfTheEyrie
 CoffeeAndPolitics