Partition of an interval

Source: Wikipedia, the free encyclopedia.
This article is about grouping elements of an interval using a sequence. For grouping elements of a set using a set of sets, see Partition of a set.
A partition of an interval being used in a Riemann sum. The partition itself is shown in grey at the bottom, with the norm of the partition indicated in red.

In

real line is a finite sequence x0, x1, x2, …, xn of real numbers
such that

a = x0 < x1 < x2 < … < xn = b.

In other terms, a partition of a compact interval I is a strictly increasing sequence of numbers (belonging to the interval I itself) starting from the initial point of I and arriving at the final point of I.

Every interval of the form [xi, xi + 1] is referred to as a subinterval of the partition x.

Refinement of a partition

Another partition Q of the given interval [a, b] is defined as a refinement of the partition P, if Q contains all the points of P and possibly some other points as well; the partition Q is said to be “finer” than P. Given two partitions, P and Q, one can always form their common refinement, denoted P ∨ Q, which consists of all the points of P and Q, in increasing order.[1]

Norm of a partition

The norm (or mesh) of the partition

x0 < x1 < x2 < … < xn

is the length of the longest of these subintervals[2][3]

max{|xi − xi−1| : i = 1, … , n }.

Applications

Partitions are used in the theory of the Riemann integral, the Riemann–Stieltjes integral and the regulated integral. Specifically, as finer partitions of a given interval are considered, their mesh approaches zero and the Riemann sum based on a given partition approaches the Riemann integral.[4]

Tagged partitions

A tagged partition[5] or Perron Partition is a partition of a given interval together with a finite sequence of numbers t0, …, tn − 1 subject to the conditions that for each i,

xi ≤ ti ≤ xi + 1.

In other words, a tagged partition is a partition together with a distinguished point of every subinterval: its mesh is defined in the same way as for an ordinary partition. It is possible to define a

partial order on the set of all tagged partitions by saying that one tagged partition is bigger than another if the bigger one is a refinement of the smaller one.[citation needed
]

Suppose that x0, …, xn together with t0, …, tn − 1 is a tagged partition of [a, b], and that y0, …, ym together with s0, …, sm − 1 is another tagged partition of [a, b]. We say that y0, …, ym together with s0, …, sm − 1 is a refinement of a tagged partition x0, …, xn together with t0, …, tn − 1 if for each integer i with 0 ≤ i ≤ n, there is an integer r(i) such that xi = yr(i) and such that ti = sj for some j with r(i) ≤ j ≤ r(i + 1) − 1. Said more simply, a refinement of a tagged partition takes the starting partition and adds more tags, but does not take any away.

See also

References

  1. ISBN 9781139458955
    .
  2. ISBN 9781441994882
    .
  3. ISBN 9783540406334
    .
  4. ISBN 9780387364254
    .
  5. ISBN 9781441969507
    .

Further reading
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