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# /qa/ - Questions and Answers

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No.85693

People say series are either convergent or divergent, but if that's true then where's the border between the two? Checkmate nerds!

No.85694

File:[MoyaiSubs] Mewkledreamy -….jpg (182.33 KB,1920x1080)

I don't know what that means...
Numbers?

No.85695

A series being convergent means it sequence of partial sums is convergent. A sequence is convergent iff it's Cauchy, meaning for every epsilon > 0 blah blah blah.

[checks Wikipedia]
>A sequence <math display="block">x_1, x_2, x_3, \ldots[/itex] of real numbers is called a Cauchy sequence if for every [[Positive and negative numbers|positive]] real number $\varepsilon,$ there is a positive [[integer]] N such that for all [[natural numbers]] $m, n > N,$ <math display="block">|x_m - x_n| < \varepsilon,[/itex] where the vertical bars denote the [[absolute value]].

If it's not convergent, there is an epsilon > 0 which is a counterexample. The "blah blah blah" is false for epsilon = 0. We can use the supremum of the epsilons for which "blah blah blah" is false to measure how divergent a sequence is (if the epsilons are not bounded, assign infinity). So we have a scale of divergence from 0 to infinity inclusive where 0 is convergent and anything above that is divergent. On this scale all the convergent stuff would be on the boundary.

Now I wonder if we can we do it the other way around, finding a number that measures how convergent a sequence is.

No.85696

>>85695
Equivalently we can use limit superior - limit inferior, taking the ∞ - ∞ and (-∞) - (-∞) cases to be ∞.

No.85697

always seemed odd to me that series like 1-1+1-1+1-1+1-1+1-1+... were called divergent

No.85698

My first thought was that you meant anime shows by series, and divergent or convergent in a sense of entertaining or tense.

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