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Per the title, if $\sum_{1}^{\infty}(a_n)^3$ diverges, does this imply that $\sum_{1}^{\infty}(a_n)$ diverges?

I'd appreciate hints (!) for dealing with this excercise.

EDIT Per the contrapositive, it is not given that $a_n$ converges absolutely, or that it is nonnegative for all $n$.

Thank you!

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Are you assuming the $a_n$ are all non-negative real? –  Geoff Robinson Jan 5 '12 at 15:15
    
@Geoff: I am not assuming that. –  ro44 Jan 5 '12 at 15:19
    
For the sums to converge, you need $|a_n| \to 0$. Thus at some point, you'll have $0 < |a_n| < 1$. Now compare $a_n$ and $a_n^3$. –  Dario Jan 5 '12 at 15:20
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Yes, prove the contrapositive. I.e. show that if $\sum a_n$ converges, $\sum a_n^3$ will converge. –  Dario Jan 5 '12 at 15:28
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@Dario: I can see why that statement is true when $a_n$ converges absolutely, however, not when all we know is that it converges. Can you explain your hint further? –  ro44 Jan 5 '12 at 15:32
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3 Answers

up vote 20 down vote accepted

In The American Mathematical Monthly, Vol. 53, No. 5, (May, 1946), pp. 283-284, you will find N. Fine's solution to G. Polya advanced problem 4142:

Let $C$ be an arbitrary subset of the positive integers ($C$ may be finite or infinite). Then there is a sequence $a_1,a_2,a_3,\dots$ of real numbers (of course, depending on $C$) such that for any positive integer $l$, $$ \sum {a_n}^{2l-1} $$ converges if and only if $l\in C$.

In particular, we can choose $C$ so that the corresponding sequence gives us an example where $\sum a_n^3$ diverges and $\sum a_n$ converges.

Two quick observations: First, in general, the $a_n$ are not going to be nonnegative. This is because if $\sum b_n$ converges then $b_n\to 0$, so if $\sum {a_n}^{2k-1}$ converges and all the $a_n$ are nonnegative, we have some $N$ such that for $n\ge N$ we have that $0\le a_n<1$. But then ${a_n}^{2l-1}={a_n}^{2k-1}{a_n}^{2(l-k)}\le {a_n}^{2k-1}$ for all $n\ge N$ and all $l\ge k$. In particular, if $\sum a_n$ converges, then so does $\sum {a_n}^3$.

Second, note that this only deals with odd exponents. This cannot be helped: For the same reason as in the previous paragraph, if $\sum {a_n}^{2k}$ converges, then so does $\sum {a_n}^{2l}$ for any $l\ge k$.

Fine's solution is constructive, by the way. Following his method, you produce an explicit example of a sequence $(a_n)$. In particular, his method gives that if $$ a_1=1, a_2=a_3=-\frac12, a_4=\frac1{\root3\of2},a_5=a_6=-\frac12\frac1{\root3\of2},a_7=\frac1{\root3\of3},\dots $$ (where for each $n$ you list $\displaystyle\frac1{\root3\of n}$ immediately followed by two $\displaystyle-\frac12\frac1{\root3\of n})$ then $\sum a_n$ converges (to 0) while $\sum {a_n}^3$ diverges (essentially because the harmonic series diverges).

In fact, in this example we have that all of $\sum {a_n}^5,\sum {a_n}^7,\dots$ converge as well.

More general patterns can be obtained if we allow the $a_n$ to be complex numbers. Variants of this problem have appeared in the Monthly a few times over the years. It would be nicer if this result were better known.

(Since this is tagged as homework, I'll leave out the details of Fine's solution.)

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Probably should have started with the explicit counter-example for this problem, since it is fairly easy to understand, although the general theorem is really nice. –  Thomas Andrews Jan 5 '12 at 16:01
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Consider a series like this: The terms are in groups of $3$. The $n$th group has two positive then one negative term: $$ \frac{1}{n^{1/3}} + \frac{1}{n^{1/3}} - \frac{2}{n^{1/3}} $$ This series then converges, but only conditionally. However, after you cube the terms, the $n$th group is: $$ \frac{1}{n} + \frac{1}{n} - \frac{8}{n} $$ So the series of cubes diverges to $-\infty$.

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hint: assume $a_n\geq0$ and $\sum a_n$ converges. then $a_n\to0$ and $0\leq a_n^3\leq a_n$ for large enough $n$ (the cube of a number between zero and one is smaller than the number itself). what does this say about $\sum a_n^3$?

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By the comparison test, it says that $a_n^3$ converges. But does this generalise for when we don't assume that $a_n\geq 0$? –  ro44 Jan 5 '12 at 15:31
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