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Partition of an interval
Partition of an interval
Partition of an interval
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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 mathematics, a partition of an interval [a, b] on the 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

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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

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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

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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

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A tagged partition 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.[5]

See also

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References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Partition of an interval

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In mathematics, particularly in , a partition of a closed interval [a,b][a, b] on the real line is a finite set of points P={x0,x1,…,xn}P = \{x_0, x_1, \dots, x_n\} such that a=x0<x1<⋯<xn=ba = x_0 < x_1 < \dots < x_n = b, which divides the interval into nn consecutive subintervals [xi−1,xi][x_{i-1}, x_i] for i=1,…,ni = 1, \dots, n. Partitions form the foundational structure for defining the , where they enable the construction of Riemann sums—approximations of the net area under a curve ff over [a,b][a, b] given by ∑i=1nf(ti)(xi−xi−1)\sum_{i=1}^n f(t_i) (x_i - x_{i-1}) for sample points ti∈[xi−1,xi]t_i \in [x_{i-1}, x_i]—that converge to the integral as the partition is refined. The norm (or mesh) of a partition PP, denoted ∥P∥\|P\| or ∣P∣|P|, is the length of its longest subinterval, max⁡1≤i≤n(xi−xi−1)\max_{1 \leq i \leq n} (x_i - x_{i-1}), providing a measure of its coarseness or fineness; finer partitions with smaller norms yield more accurate approximations. A refinement of PP is another partition QQ that includes all points of PP plus additional points, ensuring ∥Q∥≤∥P∥\|Q\| \leq \|P\| and allowing for the systematic improvement of sums in integrability criteria. Tagged partitions extend this by associating a tag tit_i with each subinterval, facilitating choices like left endpoints, right endpoints, or midpoints in Riemann sums. Partitions also underpin via upper and lower sums, where the supremum and infimum of ff on each subinterval bound the integral's value, with a function deemed Riemann integrable if these sums approach the same limit over all refinements.

Basic Concepts

Definition

In real analysis, a partition of a closed bounded interval [a,b][a, b] on the real line, where a<ba < b, is a finite ordered set of points a=x0<x1<⋯<xn=ba = x_0 < x_1 < \cdots < x_n = b, with nn a positive integer. This set divides the interval into nn consecutive subintervals [xi−1,xi][x_{i-1}, x_i] for i=1,…,ni = 1, \dots, n. Partitions are typically defined for closed bounded intervals because such intervals are compact in R\mathbb{R}, ensuring that the subintervals are also compact and that continuous functions on them attain their suprema and infima, which is crucial for applications like Riemann integration. The notation for a partition is P={x0,x1,…,xn}P = \{x_0, x_1, \dots, x_n\}, and the length of the ii-th subinterval is denoted Δxi=xi−xi−1\Delta x_i = x_i - x_{i-1}. Although the standard focus is on closed intervals, partitions of open intervals (a,b)(a, b) require adjustments, such as selecting points a<y1<y2<⋯<ym<ba < y_1 < y_2 < \cdots < y_m < b to form open subintervals (a,y1),(y1,y2),…,(ym,b)(a, y_1), (y_1, y_2), \dots, (y_m, b), excluding the endpoints to maintain openness.

Examples

A simple example of a partition of the closed bounded interval [0,1][0, 1] is P={0,0.5,1}P = \{0, 0.5, 1\}, which divides the interval into two subintervals [0,0.5][0, 0.5] and [0.5,1][0.5, 1], each of length 0.50.5. This uniform division illustrates how partitions split an interval into contiguous segments starting and ending at the specified points. For a partition with unequal subinterval lengths, consider P={0,0.2,0.7,1}P = \{0, 0.2, 0.7, 1\} of [0,1][0, 1], yielding subintervals [0,0.2][0, 0.2], [0.2,0.7][0.2, 0.7], and [0.7,1][0.7, 1] with lengths 0.20.2, 0.50.5, and 0.30.3, respectively. Such partitions demonstrate flexibility in point placement while maintaining strict increase from the left endpoint to the right. To visualize a partition, sketch the interval as a line segment on the number line, marking the partition points in strictly increasing order from left to right, and shading or labeling the resulting subintervals between consecutive points. This representation highlights the ordered, finite nature of the points and their role in segmenting the interval. Although the primary focus is on bounded closed intervals, partitions can extend to unbounded intervals like [0,∞)[0, \infty) by using sequences of points approaching infinity, such as subintervals [n,n+1][n, n+1] for n=0,1,2,…n = 0, 1, 2, \dots, forming an infinite collection of bounded segments. These examples can be refined by inserting additional points between existing ones to create finer divisions.

Properties

Refinements

A refinement of a partition P={x0,x1,…,xn}P = \{x_0, x_1, \dots, x_n\} of the interval [a,b][a, b] is another partition Q={y0,y1,…,ym}Q = \{y_0, y_1, \dots, y_m\} such that every point in PP is also in QQ, meaning Q⊇PQ \supseteq P as sets of points, with the points of QQ including all of PP and possibly additional points strictly between some consecutive points of PP. Note that any partition is a refinement of itself, since it contains all its own points. Refinements always exist for any given partition; for instance, one can insert additional points between existing consecutive partition points to form a finer partition. When refining a partition, the lengths of the subintervals in the new partition are either reduced (for those subintervals that are split) or maintained (if no points are added within them), ensuring that no subinterval length increases. As an example, consider the partition P={0,1}P = \{0, 1\} of [0,1][0, 1]; a refinement Q={0,0.25,0.5,0.75,1}Q = \{0, 0.25, 0.5, 0.75, 1\} can be constructed by inserting the midpoints of the original subinterval. The set of all partitions of [a,b][a, b] forms a partially ordered set (poset) under the refinement relation, where P≤QP \leq Q if QQ is a refinement of PP, meaning finer partitions are greater in the order. In this poset, the coarsest (minimal) element is the trivial partition {a,b}\{a, b\}, which has no further coarsenings. A key property of this poset is that any two partitions PP and QQ of [a,b][a, b] have a common refinement, obtained by taking the union of the points in PP and QQ, sorting them in increasing order, and removing any duplicates to form a new partition R⊇P∪QR \supseteq P \cup Q. This common refinement RR is finer than both PP and QQ, and the construction ensures the poset has the property that every pair of elements has an upper bound.

Norm

The norm of a partition P={x0,x1,…,xn}P = \{x_0, x_1, \dots, x_n\} of a closed interval [a,b][a, b], where a=x0<x1<⋯<xn=ba = x_0 < x_1 < \dots < x_n = b, is defined as the maximum length of its subintervals, denoted ∥P∥=max⁡1≤i≤n(xi−xi−1)\|P\| = \max_{1 \leq i \leq n} (x_i - x_{i-1}). This measure quantifies the coarseness or fineness of the partition, with smaller norms indicating finer divisions of the interval. A key property of the norm is its behavior under refinement: if QQ is a refinement of PP (obtained by adding points to PP), then ∥Q∥≤∥P∥\|Q\| \leq \|P\|, since subdividing subintervals can only reduce or maintain the maximum length. Repeated refinements can make the norm arbitrarily small; specifically, for any ϵ>0\epsilon > 0, there exists a refinement whose norm is less than ϵ\epsilon, allowing partitions to approximate continuous structures with increasing precision. Uniform partitions represent a special case where all subintervals have equal length Δx=(b−a)/n\Delta x = (b - a)/n, so the norm is simply ∥P∥=(b−a)/n\|P\| = (b - a)/n. For example, consider the partition P={0,0.2,0.7,1}P = \{0, 0.2, 0.7, 1\} of [0,1][0, 1]; the subinterval lengths are 0.2−0=0.20.2 - 0 = 0.2, 0.7−0.2=0.50.7 - 0.2 = 0.5, and 1−0.7=0.31 - 0.7 = 0.3, yielding ∥P∥=0.5\|P\| = 0.5. In , the norm plays a crucial role in convergence arguments, where sequences of partitions with ∥Pk∥→0\|P_k\| \to 0 as k→∞k \to \infty ensure that associated sums, such as Riemann sums, approach their limits uniformly. This property underpins the definition of integrability, as functions are Riemann integrable if the difference between upper and lower sums vanishes for sufficiently small norms.

Variants

Tagged Partitions

A tagged partition of a closed interval [a,b][a, b] extends the concept of a standard partition by associating a specific point, called a tag, within each subinterval for evaluating functions. Formally, given a partition P={a=x0<x1<⋯<xn=b}P = \{a = x_0 < x_1 < \dots < x_n = b\} of [a,b][a, b], a tagged partition is the pair (P,{ti}i=1n)(P, \{t_i\}_{i=1}^n), where each tag tit_i satisfies xi−1≤ti≤xix_{i-1} \leq t_i \leq x_i for i=1,…,ni = 1, \dots, n. The tags can be chosen arbitrarily within their respective subintervals, such as at the endpoints or midpoints, allowing flexibility in construction. For instance, consider the interval [0,1][0, 1] with partition P={0,0.5,1}P = \{0, 0.5, 1\}; a possible tagging is t1=0.25∈[0,0.5]t_1 = 0.25 \in [0, 0.5] and t2=0.75∈[0.5,1]t_2 = 0.75 \in [0.5, 1], yielding the tagged partition ((0,0.5,1),{0.25,0.75})((0, 0.5, 1), \{0.25, 0.75\}). These tags enable the evaluation of a function ff at precise points in each subinterval, facilitating the formation of sums like ∑i=1nf(ti)(xi−xi−1)\sum_{i=1}^n f(t_i) (x_i - x_{i-1}). When refining a tagged partition, one starts with a refinement QQ of the underlying partition PP (obtained by adding points to PP) and selects new tags in the resulting subintervals, ensuring the structure is preserved while potentially reducing the overall mesh. Common variations of tagged partitions specify the tag positions systematically. In a left-tagged partition, each ti=xi−1t_i = x_{i-1}; in a right-tagged partition, ti=xit_i = x_i; and in a midpoint-tagged partition, ti=xi−1+xi2t_i = \frac{x_{i-1} + x_i}{2}. These choices often simplify computations for specific functions or approximations. The norm of a tagged partition coincides with that of its underlying partition PP, defined as the maximum length of the subintervals, ∥(P,{ti})∥=max⁡1≤i≤n(xi−xi−1)\| (P, \{t_i\}) \| = \max_{1 \leq i \leq n} (x_i - x_{i-1}), independent of the tags selected.

Measurable Partitions

In measure theory, particularly within the framework of on the real line, a measurable partition of a bounded interval [a,b][a, b] generalizes classical partitions by allowing arbitrary measurable sets while requiring that the σ-algebra generated by the partition (consisting of its saturated sets) is countably generated, or more precisely, that the partition is equivalent modulo null sets to one satisfying this condition. This ensures compatibility with the standard Borel structure of the interval and accommodates advanced applications beyond Riemann integration, such as in . A key property of such partitions is that they have at most countably many elements of positive Lebesgue measure, as the measure is σ-additive and the total measure b−ab - a is finite. For the standard Lebesgue measure, subintervals remain measurable, preserving Borel measurability. Finite point-based partitions of [a,b][a, b] naturally induce measurable partitions by forming the subintervals between consecutive points, each of which is a measurable set under Lebesgue measure. These measurable partitions extend the toolkit for handling non-continuous functions and infinite processes, finding application in ergodic theory where they facilitate the decomposition of measure spaces into factors and the study of dynamical systems on intervals.

Applications

Riemann Integration

The concept originates from Bernhard Riemann's 1854 memoir on trigonometric series, where partitions were introduced to define the integral via limits of sums. In the context of Riemann integration, partitions of an interval [a,b][a, b] serve as the foundation for approximating the integral of a bounded function f:[a,b]→Rf: [a, b] \to \mathbb{R} through Riemann sums. For a tagged partition (P,{ti})(P, \{t_i\}) of [a,b][a, b], where P={x0=a,x1,…,xn=b}P = \{x_0 = a, x_1, \dots, x_n = b\} and ti∈[xi−1,xi]t_i \in [x_{i-1}, x_i] for each i=1,…,ni = 1, \dots, n, the corresponding Riemann sum is defined as ∑i=1nf(ti)Δxi\sum_{i=1}^n f(t_i) \Delta x_i, with Δxi=xi−xi−1\Delta x_i = x_i - x_{i-1}. This sum represents an approximation of the area under the curve of ff by summing the areas of rectangles with heights f(ti)f(t_i) and widths Δxi\Delta x_i. A function ff is Riemann integrable on [a,b][a, b] if the limit of these Riemann sums exists and is the same value II, regardless of the choice of tags or the specific partitions used, as the norm ∥P∥\|P\| (the maximum length of the subintervals) approaches zero. Formally, for every ϵ>0\epsilon > 0, there exists δ>0\delta > 0 such that for any tagged partition (P,{ti})(P, \{t_i\}) with ∥P∥<δ\|P\| < \delta, ∣∑i=1nf(ti)Δxi−I∣<ϵ\left| \sum_{i=1}^n f(t_i) \Delta x_i - I \right| < \epsilon. This limit defines the Riemann integral ∫abf(x) dx=I\int_a^b f(x) \, dx = I. The independence from tags and refinements ensures the integral is well-defined for suitable functions. An equivalent formulation, known as the Darboux integral, uses upper and lower sums without tags to characterize integrability. For a partition PP, the upper sum is U(f,P)=∑i=1n(sup⁡[xi−1,xi]f)ΔxiU(f, P) = \sum_{i=1}^n (\sup_{[x_{i-1}, x_i]} f) \Delta x_i and the lower sum is L(f,P)=∑i=1n(inf⁡[xi−1,xi]f)ΔxiL(f, P) = \sum_{i=1}^n (\inf_{[x_{i-1}, x_i]} f) \Delta x_i. Any Riemann sum for (P,{ti})(P, \{t_i\}) satisfies L(f,P)≤∑i=1nf(ti)Δxi≤U(f,P)L(f, P) \leq \sum_{i=1}^n f(t_i) \Delta x_i \leq U(f, P). The function ff is Riemann integrable if the upper integral inf⁡PU(f,P)\inf_P U(f, P) equals the lower integral sup⁡PL(f,P)\sup_P L(f, P), or equivalently, if for every ϵ>0\epsilon > 0, there exists a partition PP such that U(f,P)−L(f,P)<ϵU(f, P) - L(f, P) < \epsilon. Refinements of partitions decrease the difference U(f,P)−L(f,P)U(f, P) - L(f, P), facilitating convergence. To illustrate, consider f(x)=xf(x) = x on [0,1][0, 1], which is continuous and thus Riemann integrable. For a uniform partition Pn={0,1/n,2/n,…,1}P_n = \{0, 1/n, 2/n, \dots, 1\} with midpoint tags tk=(k−1/2)/nt_k = (k - 1/2)/n for k=1,…,nk = 1, \dots, n, the Riemann sum is S(f,Pn,{tk})=∑k=1nf(tk)⋅1n=1n∑k=1nk−1/2n=1n2∑k=1n(k−12)=1n2(n(n+1)2−n2)=1n2⋅n22=12.S(f, P_n, \{t_k\}) = \sum_{k=1}^n f(t_k) \cdot \frac{1}{n} = \frac{1}{n} \sum_{k=1}^n \frac{k - 1/2}{n} = \frac{1}{n^2} \sum_{k=1}^n \left(k - \frac{1}{2}\right) = \frac{1}{n^2} \left( \frac{n(n+1)}{2} - \frac{n}{2} \right) = \frac{1}{n^2} \cdot \frac{n^2}{2} = \frac{1}{2}. This sum equals 1/21/2 exactly for every nn, converging to ∫01x dx=1/2\int_0^1 x \, dx = 1/2 as ∥Pn∥=1/n→0\|P_n\| = 1/n \to 0. Continuous functions on the compact interval [a,b][a, b] are always Riemann integrable, as their uniform continuity ensures that U(f,P)−L(f,P)U(f, P) - L(f, P) can be made arbitrarily small by choosing partitions with sufficiently small norm. However, discontinuities can prevent integrability; for bounded functions, Riemann integrability holds the set of discontinuities has zero, though functions with finitely many discontinuities may still fail if unbounded.

Other Uses

In numerical methods for approximating definite integrals, partitions of an interval are essential for composite rules such as the and , where the interval is divided into subintervals to apply local approximations and sum the results for a global estimate. The approximates the integral over each subinterval using a trapezoid formed by connecting function values at the endpoints, while employs a parabolic arc fitted to three points (the endpoints and ) for higher-order accuracy. Error bounds for these methods depend on the norm of the partition, defined as the length of the longest subinterval, with finer partitions (smaller norms) yielding tighter error estimates proportional to the norm squared for the and the fourth power for . In , particularly for processes, partitions of time intervals like [0, T] are used to approximate paths of processes such as , enabling the construction of integrals and through sums over subintervals. For instance, as the of the partition (maximum subinterval length) approaches zero, these sums converge in probability to the of , which equals time itself for standard Wiener processes. This discretization facilitates simulations and theoretical analysis of continuous-time processes by breaking them into manageable discrete steps. In optimization, interval partitions arise in dynamic programming algorithms designed to find optimal divisions of a continuous or discrete interval to minimize a cost function, such as in resource allocation or data segmentation problems. These methods compute solutions recursively by considering all possible ways to split the interval at intermediate points, using a fitness function to evaluate partition quality, and achieve polynomial-time complexity for many instances. Refinements of such partitions can iteratively improve optimality by adjusting splits based on local criteria. In , discretizing time intervals via partitions supports the application of transforms like the (DFT) and (DWT), converting continuous signals into discrete sequences for frequency-domain analysis. The DFT partitions the into equally spaced samples to compute spectral components efficiently via the algorithm, while the DWT uses dyadic partitions (halving intervals successively) to provide multi-resolution analysis, capturing both time localization and frequency content better than fixed-window methods. A practical example appears in for ray tracing, where bounding interval hierarchies partition the parametric interval along each ray into subintervals to accelerate intersection tests with scene geometry, reducing computational overhead by quickly non-intersecting regions. further enhances this by propagating bounds over partitions to verify intersections with implicit surfaces, ensuring robustness against floating-point errors. Despite these benefits, finer partitions in such applications often increase computational cost linearly or quadratically with the number of subintervals, trading off accuracy against efficiency; for example, in or ray tracing, excessive refinement can lead to prohibitive runtime without proportional gains in precision.

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