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Decreasing sequence of non-negative integers

Mathematics Asked on November 18, 2021

Let $ a_n , n=0, 1, 2, dots $ be a strictly monotonically decreasing sequence consisting of non-negative integers. The sequence also has a lower bound C such that $ a_n ge C >-infty.$

Intuitively, we know, that $a_n$ has to have finite amount of entries, but can this be proven exactly?

2 Answers

Well, this seems kind of obvious, but let's do it anyway. We know three things:

1: The sequence is strictly decreasing, i.e, $a_{n+1}<a_n.$

2: $a_n in mathbb{Z} ~~forall n.$

3: $a_ngeq 0 ~forall n$.

Condition (1) implies that all of the terms of $a_n$ are distinct, i.e, $nexists m,ninmathbb{N}:a_m=a_n$. Now using condition (2), let $a_0=N$, some positive integer. We know, using condition (1), that all subsequent terms are $<N$, and using condition (3), are all $geq0.$ Now suppose $a_n$ had infinitely many terms. That would imply that there are infinitely many integers in the range $[0,N]$, which is clearly false. Thus $a_n$ can only have finitely many terms.

Answered by K.defaoite on November 18, 2021

Can we ever have this kind of sequence ? As it is strictly monotonically decreasing so we have $a_1 geq a_n geq 0$ for all $ ngeq 2 $ but there are only finitely many non negative integers satisfying this .so the sequence is eventually constant which contradicts the fact it is strictly monotonically decreasing.

Answered by Subhajit Ghosh on November 18, 2021

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