The limit definition of a definite integral

Let $f(x)$ be a continuous function on the interval $[a,b]$, $(a<b)$ and let: $$a=x_0,x_1,x_2,...,x_{i-1},x_i,...,x_{n-1},x_n=b$$ be an arbitrary (randomly selected) partition of the interval $[a, b]$ , which divides the interval into $n$ subintervals (subdivisions). Let

$c_{1}, c_{2}, c_{3},$ ... $, c_{n-2}, c_{n-1}, c_{n}$

be the sampling numbers (or sampling points) selected from the subintervals. That is,

$c_{1}$ is in $[x_{0}, x_{1}], c_{2}$ is in $[x_{1}, x_{2}], c_{3}$ is in $[x_{2}, x_{3}],..., c_{n-2}$ is in $[x_{n-3}, x_{n-2}], c_{n-1}$ is in $[x_{n-2}, x_{n-1}]$ and $c_{n}$ is in $[x_{n-1}, x_{n}]$ .

Define the mesh of the partition to be the length of the largest subinterval. That is, let

$\Delta x_{i} = x_{i} - x_{i-1} \ \$

for $i = 1, 2, 3, ..., n$ and define

$mesh = \displaystyle{ \max_{1 \le i \le n} \{ x_{i} - x_{i-1} \}}$ .

The definite integral of $f$ on the interval $[a, b]$ is most generally defined to be

$$\displaystyle{ \int^{b}_{a} f(x) \, dx} \overset{def}{=} \displaystyle{ \lim_{mesh \to 0} \sum_{i=1}^{n} f(c_{i}) \Delta x_{i} }$$

A special case of the above definition uses $n$ subintervals of equal length and sampling points chosen to be the right-hand endpoints of the subintervals. Thus, each subinterval has length

$\ \ \ \ \ \ \ \ \Delta x_{i} = \displaystyle{ b-a \over n }$

for $i = 1, 2, 3, ..., n$ and the right-hand endpoint formula is

$\ \ \ \ \ \ \ \ c_{i} = \displaystyle{ a + \Big( { b-a \over n } \Big) i }$

for $i = 1, 2, 3, ..., n$ . The definite integral of $f$ on the interval $[a, b]$ can now be alternatively defined by

$$\displaystyle{ \int^{b}_{a} f(x) \, dx} \overset{def}{=} \displaystyle{ \lim_{n \to \infty} \sum_{i=1}^{n} f(c_{i}) \Delta x_{i} }$$

The part I don't understand is why the second definition is a special case for $n$ subintervals of equal length. Why does their length matter if $n$ approaches infinity?

• In the first definition, we have the limit of a Riemann sum where the intermediate points are arbitrary within each subinterval and the partition points are not necessarily evenly spaced. This is guaranteed to converge for a Riemann integrable function -- which you have since $f$ is continuous and bounded. This implies the limit in the second definition converges to the same value.
– RRL
Mar 9, 2016 at 18:02
• It can also be shown for that for a continuous function that convergence of the second definition limit implies convergence of the first, but this takes some effort. Look up Cauchy integrable versus Riemann-Darboux integrable.
– RRL
Mar 9, 2016 at 18:04

It's relevant because if the intervals are all of equal length, we then know the length of any given interval. We can then use that to actually calculate $c_i$; with that, we can take the limit of the Riemann sum to calculate the definite integral without having to use the fundamental theorem of calculus.
• Why doesn't the length of subintervals matter in the first case, when we define the definite integral as a limit when $mesh \to 0$?