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.
| Behaviour | Description |
|---|---|
| Increasing sequence | Each term is greater than or equal to the preceding term. |
| Decreasing sequence | Each term is less than or equal to the preceding term. |
| Constant sequence | All terms are the same, for example \(3,3,3,3,\ldots\). |
| Alternating sequence | The 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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