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This is actually a question I'm asking for an algorithm I write, but I think that this is the right place for the question.

I know that the definition of a descending sequence is that for every $n$:
$a_n > a_{n+1}$

A single number does not apply to the definition, does it mean that a single number isn't a descending series by definition?

Edit: The task asks me to write a function that gets a natural number, and outputs true if the digits of the number are a descending series, or false if not. I'm trying to think of radical possible inputs.

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    jeez, I'm confused @_o2012-11-27

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Precisely: def. a finite sequence $a_1,..,a_N$ is descending, iff $\forall n\in\{1,..,N-1\}$ we have $a_n>a_{n+1}$.

Then, for $N=1$, $n$ is coming from the empty set, hence the $\forall n\in\emptyset:\dots$ becomes true.