Real Null Sequence/Examples/n^alpha x^n
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Example of Real Null Sequence
Let $\alpha \in \Q$ be a (strictly) positive rational number.
Let $x \in \R$ be a real number such that $\size x < 1$.
Let $\sequence {a_n}_{n \mathop \ge 1}$ be the real sequence defined as:
- $\forall n \in \Z_{>0}: a_n = n^\alpha x^n$
Then $\sequence {a_n}$ is a null sequence:
- $\ds \lim_{n \mathop \to \infty} n^\alpha x^n = 0$
Proof
First a lemma:
Lemma
There exists $N \in \N$ such that:
- $\paren {1 + \dfrac 1 N}^{\alpha + 1} \, \size x \le 1$
$\Box$
Let $x \ne 0$.
Consider the expression:
- $\size {\dfrac {\paren {n + 1}^{\alpha + 1} x^{n + 1} } {n^{\alpha + 1} x^n} } = \paren {1 - \dfrac 1 n}^{\alpha + 1} \size x$
For all $n \ge N$, we have:
- $\size {\paren {n + 1}^{\alpha + 1} x^{n + 1} } \le \size {n^{\alpha + 1} x^n}$
Hence for all $n \ge N$:
- $\size {n^{\alpha + 1} x^n} \le \size {N^{\alpha + 1} x^n}$
So for $n \ge N$:
- $\size {n^\alpha x^n} \le \dfrac 1 N \size {N^{\alpha + 1} x^N}$
The result follows from the Squeeze Theorem for Real Sequences.
$\blacksquare$
Sources
- 1977: K.G. Binmore: Mathematical Analysis: A Straightforward Approach ... (previous) ... (next): $\S 4$: Convergent Sequences: Exercise $\S 4.20 \ (5)$