Subsequence – Serlo

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Introductory Example[Bearbeiten]

Einführendes Beispiel zu Teilfolgen (YouTube-Video vom YouTube-Kanal "MJ Education“)

Sometimes its necessary to speak about the "subsequence" of a sequence. Much like a subset is a "part" of a set, a subsequence is some part of a sequence -- all elements of a subsequence are also elements of the original sequence. A subsequence is in end effect constructed by removing some chosen terms of the original sequence. Regardless of how many terms are stripped from the original sequence, the resulting subsequence still has an infinite number of terms. For example, let's take the sequence :

We are interested in a subsequence composed of every other term of the original sequence. This subsequence arises from either removing all terms with an even index or removing all terms with an odd index. If, for example, we remove all terms with odd index, we get the following schematic:

This gives rise to a subsequence that is constant .

Mathematical Description[Bearbeiten]

Erklärung der mathematischen Schreibweise für Teilfolgen (YouTube-Video vom YouTube-Kanal "MJ Education")

How can subsequences be denoted? First let's look at the indices of the sequence terms that we want to keep in the subsequence:

Now we want to find a sequence that describes these indices. In the above example we consider even indices. So the sequence can be written as :

We substitute this sequence into . From there we get the subsequence :

First we will build the sequence of relevant indices of the subsequence. We will set this subsequence into the original sequence for so that we get the sequence .

In our example we have . So we substitute for in . Then we obtain the subsequence .

Definition[Bearbeiten]

Definition (Subsequence)

Let be an arbitrary sequence. Any sequence is called a subsequence of if is a strictly monotone increasing sequence of natural numbers.

This concept is important for analysis since it is used to characterize so-called "limit points." However, these will not be properly defined and discussed until the next chapter.

Hint

Every sequence is a subsequence of itself. Namely, if one chooses , then . For the subsequence is identical to the original sequence . This shows that every sequence is a subsequence of itself.

Exercise (Subsequences)

Find five different subsequences of the sequence .

Solution (Subsequences)

The sequence has infinitely many subsequences. Five examples would be

Convergence of Subsequences[Bearbeiten]

For subsequences we have the following important theorem:

Theorem (Convergence of Subsequences)

Let be a sequence. It converges if and only if every subsequence converges. The limit of the sequence coincides with the limits of the subsequences.

Proof (Convergence of Subsequences)

To prove the equivalence

we have to prove the following two implications:

  1. If converges to , then every subsequence of converges to , as well.
  2. If every subsequence of converges to , then converges to , as well.

We can split the proof into these two parts, since the statement is equivalent to the two implications .

Proof step: Convergence of the sequence implicates the convergence of all subsequences - to the same limit.

Let be a sequence converging to .Now, we need to prove that all subsequences of converge to , as well.

So, let be a subseqeunce of . We would like to show that converges to . Let be given arbitrarily. Since is the limit of , there must be an index , such that for all the inequality holds.

Since the sequence is by definition strictly decreasing , there will be for all . Therefore, for all , since from and we conclude . Hence, there is also for all .

We have just proven that for any given there is an with for all . But this is just the definition for " converges to " . Since any subsequence can be chosen in this place, the proof holds for all subsequences of .

Proof step: Convergence of all subsequences to the same limit implies convergence of the sequence.

We know that all subsequences of converge to . But now, the sequence is a subsequence of itself (just set ). So the sequence itself must also converge to .

Example (Convergence of subsequences)

Since is a null sequence, the same will hold for the two limits

Hint

The above theorem directly implies that a convergent sequence does not change its limit, if a finite number of elements are canceled. Removing a finite amount of elements will render a subsequence of . And we just showed that the limits of those two sequences coincide.

The above theorem directly implies:

Theorem (Divergence in case of a diverging subsequence)

If a subsequence diverges, the original sequence must also diverge.

Check your understanding: Why does the above theorem imply that the original sequence has to diverge, if any subsequence does so?

We know: If a sequence converges, then all of its subsequences also have to diverge. If there was a convergent sequence for which we could find a divergent subsequence, we would have a contradiction to the theorem. So if a subsequence diverges, we know that the original sequence must have been convergent.

Example (DDivergence of subsequences)

Let us consider the sequence with . A subsequence is obtained by selecting only the positive elements, which are . Since is an unbounded sequence, it diverges. Therefore, the original sequence must also diverge.

Application: convergence of mixed sequences[Bearbeiten]

In the chapter „Beispiele und Eigenschaften von Folgen“ we have seen how to compose to sequences and to a "mixed sequence" . This mixture is defined as

That means, the sequence is composed out of the two subsequences and .

We may now ask how the convergence of the mixture relates to the convergences of its two constituents and . In order for to converge, two conditions must be satisfied:

  • First, both subsequences and have to converge, as we know that for convergent sequences, all subsequences converge.
  • Second, the limits of and must be identical. This is because if converges, then all of its subsequences must tend to the same limit.

If one of these two conditions is not satisfied, the mixed sequence must diverge. But are the two conditions also sufficient for convergence of the mixed sequence? Indeed, they are! We will now proof this. The limit of the mixed sequence must the coincide with the two limits of the subsequences.

Theorem (Convergence of mixed sequences)

Let and be two sequences and a limit candidate. Let the mixed sequence be defined by
. It converges to , if and only if the sequences and both converge to .

Proof (Convergence of mixed sequences)

Proof step: If converges to , then both and converge to .

As and , both and are subsequences of . Since converges to , all of its subsequences will do so, too. This specifically includes and , which hence converge to .

Proof step: If both and converge to , then also converges to .

As both and converge to , the following two statements hold:

Since and there is:

Let now be given arbitrarily. The above statements imply that there is a threshold number (for b) with for all . In addition, there is a threshold (for c) with for all . We choose the maximum of both thresholds . Let be a number above this threshold. If is odd, then for some . Since , there is especially and hence

In case is even, we know that for some . As , there is and therefore

In any case, for all . This establishes convergence .

Check your understanding: In the above proof, we chose . Why couldn't we have chosen  ?

In the above proof, we used that for all . This means, starting from a sequence element , all odd elements will satisfy the inequality . Analogously, for all implies that starting from a sequence element all even elements will satisfy the inequality . I order to prove , we therefore need and . Hence, we choose , so both ensures and .

Example (Convergence of mixed sequences)

Consider the mixed sequence with . This can be considered a mixed sequence with

The two constituents of are the subsequences and . Now

Both subsequences converge to zero. For the above theorem, the mixed sequence must therefore also converge to zero.