IF A is a non-empty bounded set of real numbers with least upper bound L and p is a positive real number, then the set pA has least upper bound pL.
definition i have to use:
The number q is an UPPER BOUND for the set A if x<=q for all x in A.
The LEAST UPPER BOUND of the nonempty set A is L if L is an upper bound for A and if r is a number less than L, then there is a member a in A so that a > r.
what i have so far.
Since A is non empty and the least upper bound of A is L, then a<=L for all a in A.
Since a<=L for all a in A and p is positive, then pa<=pL.
therefor pL is a upper bound of pA.
now im lost on the second part of this problem where i need to prove that pL is the least upper bound of pA. my initial thought was to just use the same trick as above but this feels flawed for some reason.
since L is the least upper bound of A, then r < a <=L where r is a number less than L.
since r < a <=L where r is a number less than L, and p is positive, then rp < pa <= pL.
since rp < pa <= pL, and rp is a number less than pL, then pL is the least upper bound of pA.
so the full proof should be.
Let , , , , and p be a positive number.
Since , and , then .
Since , then .
since , and , then .
since , then
since , and , then
since , then . a contradiction as must be true for
I taught analysis for more than 35 years.
I drilled into students that if then for any then it must be the case that .
Oddly enough, that turns out to be the most important idea in analysis.
So if because that must be a contradiction because