# Correct this improper definition of a limit

by Easy_as_Pi
Tags: correct, definition, improper, limit
 P: 31 1. The problem statement, all variables and given/known data Eddy wrote on his midterm exam that the definition of the limit is the following: The sequence {an} converges to the real number L if there exists an N ∈ Natural numbers so that for every $\epsilon$ > 0 we have |an − L| < $\epsilon$ for all n > N. Show Eddy why he is wrong by demonstrating that if this were the definition of the limit then it would not be true that lim n→∞ 1/n = 0. (Hint: What does it mean if |a − b| < $\epsilon$ for every $\epsilon$ > 0?) 2. Relevant equations |a-b| <ε means that ||a|-|b|| < ε from the reverse triangle inequality 3. The attempt at a solution I know it has to do with the fact that the actual definition of a limit has "for every ε > 0, there exists an N $\in$ Natural numbers S.T. ...." so, Eddy reversed that part of the definition. I just haven't been able to quite see the difference of the two. A little push in the right direction would be greatly appreciated. I like figuring these out on my own, so no full on answers, please.
 PF Patron Sci Advisor Thanks Emeritus P: 15,671 So there exists a certain N. Let [itex]\varepsilon = |1/N|[/tex] and try to prove that 1/n does not converge to 0 with this definition.
 P: 31 I'm not quite sure I follow. I ended up answering it this way: Eddy's definition implies there is a single natural number, N, such that for all n>N |1/n|< every epsilon greater than zero. Which is not true. For every epsilon you give me, I can find an N such that 1/n is less than that epsilon for all n>N, but if you pick a newer, smaller epsilon, my N has to be larger, and since the natural numbers are unbounded, we can do this forever. But, it's a different N for each new epsilon, not one single N like eddy implied. Does that make sense?
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