# Greatest integer and modulus function limit?

• Jun 9th 2009, 08:04 AM
fardeen_gen
Greatest integer and modulus function limit?
Evaluate:
$\lim_{x\rightarrow 0^{-}} \frac{\sum_{r = 1}^{2n + 1} [x^r] + (n + 1)}{1 + [x] + |x| + 2x}$

where $[.]$ denotest the greatest integer function.

• Jun 10th 2009, 03:57 PM
the_doc
I don't believe this function's limit can be defined from the left as the greatest integer function jumps from when x is just less than 0 to when it is 0.
• Jun 11th 2009, 03:53 AM
Moo
Quote:

Originally Posted by the_doc
I don't believe this function's limit can be defined from the left as the greatest integer function jumps from when x is just less than 0 to when it is 0.

Yet it is still defined as x doesn't technically attain 0... ?
• Jun 11th 2009, 05:47 AM
the_doc
Had a look at the question again and you're correct Moo - I misread. It does exist as we only need to get arbitrarily close to x=0 to define the limit.

In which case we can consider the range $-1 < x < 0$ where

$-1 < x^r < 0$ for $r$ odd $\Rightarrow [x^r ] = -1$
$0 < x^r < 1$ for $r$ even $\Rightarrow [x^r ] = 0

$
so within this range

$\sum_{r=1}^{2n+1} [x^r ] = -(n+1) .$

Also in this range we have that

$1 + [x] + |x| + 2x = 1 -1 -x + 2x = x$

and so since

$\frac{0}{x} = 0$ when $x<0
$

$\lim_{x \to 0^{-}} \frac{\sum_{r = 1}^{2n + 1} [x^r] + (n + 1)}{1 + [x] + |x| + 2x}= 0$