Let a closed interval be partitioned by points
, where the lengths of
the resulting intervals between the points are denoted
,
, ...,
. Let
be an arbitrary point in the
th subinterval. Then the quantity
is called a Riemann sum for a given function and partition, and the value
is called the mesh size
of the partition.
If the limit of the Riemann sums exists as , this limit is known as the Riemann integral
of
over the interval
. The shaded areas in the above plots show the lower
and upper sums for a constant mesh
size.