Limits and Behaviour of Sequences

Limits and Behaviour of Sequences

TGM Original Questions

Limit of a Sequence and Behaviour of a Sequence

The limit of a sequence \(\{a_n\}\) as \(n\) approaches infinity is the value \(L\) that the terms of the sequence get arbitrarily close to as \(n\) becomes sufficiently large.

\[\lim_{n\to\infty}a_n=L\qquad\text{or}\qquad a_n\to L\text{ as}\hspace{0.5em}n\to\infty\]

If such a finite \(L\) exists, the sequence converges to \(L\). If no such \(L\) exists, the sequence diverges.

When describing the behaviour of a sequence, first determine whether the sequence is convergent or divergent.

In addition to convergence and divergence, consider whether the sequence is increasing, decreasing, alternating or constant.

BehaviourDescription
Increasing sequenceEach term is greater than or equal to the preceding term.
Decreasing sequenceEach term is less than or equal to the preceding term.
Constant sequenceAll terms are the same, for example \(3,3,3,3,\ldots\).
Alternating sequenceThe terms alternate in sign or exhibit another regular alternation, for example \(-1,1,-1,1,\ldots\).

Take note: An alternating sequence can have terms whose signs alternate between positive and negative. A factor \((-1)^n\) is commonly used to encode alternating signs in an explicit formula. For a recurrence that reverses the sign at every step, use \(u_{n+1}=-u_n\).

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Answer:The sequence converges to \(L=8.1\).

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