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{{Short description|Pair of positions in a sequence where two elements are out of sorted order}}

[[File:Inversion qtl1.svg|thumb|Permutation with one of its inversions highlighted<br><br>It may be denoted by the pair of places (2, 4) or the pair of elements (5, 2).]]

In [[computer science]] and [[discrete mathematics]], a sequence has an '''inversion''' where two of its elements are out of their natural [[total order|order]].

An inversion may be denoted by the pair of places (2, 4) or the pair of elements (5, 2).

The inversions of this permutation using element-based notation are: (3, 1), (3, 2), (5, 1), (5, 2), and (5,4).]]

In [[computer science]] and [[discrete mathematics]], a sequence has an '''inversion''' wherein twoa ofsequence itsis a pair of elements that are out of their natural [[total order|order]].

== Definitions ==

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===Inversion===

Let <math>\pi</math> be a [[permutation]].

LetThere is an '''inversion''' of <math>\pi</math> bebetween a<math>i</math> [[permutation]].and <math>j</math> Ifif <math>i < j</math> and <math>\pi(i) > \pi(j)</math>,. either theThe inversion is indicated by an ordered pair ofcontaining either the places <math>(i, j)</math>{{sfn|Aigner|2007|pp=27}}{{sfn|Comtet|1974|pp=237}} or the pair of elements <math>\bigl(\pi(i), \pi(j)\bigr)</math>.{{sfn|Knuth|1973|pp=11}}{{sfn|Pemmaraju|Skiena|2003|pp=69}}{{sfn|Vitter|Flajolet|1990|pp=459}} is called an '''inversion''' of <math>\pi</math>.

The [[#Example:_All_permutations_of_four_elements|inversion set]] is the set of all inversions. A permutation's inversion set using place-based notation is the same as the [[Permutation#Definition|inverse permutation's]] inversion set using element-based notation with the two components of each ordered pair exchanged. Likewise, a permutation's inversion set using element-based notation is the same as the inverse permutation's inversion set using place-based notation with the two components of each ordered pair exchanged.{{sfn|Gratzer|2016|pp=221}}

The inversion is usually defined for permutations, but may also be defined for sequences:<br>

Let <math>S</math> be a [[sequence]] (or [[multiset]] permutation{{sfn|Bóna|2012|pp=57}}). If <math>i < j</math> and <math>S(i) > S(j)</math>, either the pair of places <math>(i, j)</math>{{sfn|Bóna|2012|pp=57}}{{sfn|Cormen|Leiserson|Rivest|Stein|2001|pp=39}} or the pair of elements <math>\bigl(S(i), S(j)\bigr)</math>{{sfn|Barth|Mutzel|2004|pp=183}} is called an inversion of <math>S</math>.

Inversions are usually defined for permutations, but may also be defined for sequences:<br>Let <math>S</math> be a [[sequence]] (or [[multiset]] permutation{{sfn|Bóna|2012|pp=57}}). If <math>i < j</math> and <math>S(i) > S(j)</math>, either the pair of places <math>(i, j)</math>{{sfn|Bóna|2012|pp=57}}{{sfn|Cormen|Leiserson|Rivest|Stein|2001|pp=39}} or the pair of elements <math>\bigl(S(i), S(j)\bigr)</math>{{sfn|Barth|Mutzel|2004|pp=183}} is called an inversion of <math>S</math>.

For sequences inversions according to the element-based definition are not unique, because different pairs of places may have the same pair of values.

For sequences, inversions according to the element-based definition are not unique, because different pairs of places may have the same pair of values.

The '''inversion set''' is the set of all inversions. A permutation's inversion set according to the place-based definition is that of the [[Permutation#Product and inverse|inverse]] permutation's inversion set according to the element-based definition, and vice versa,{{sfn|Gratzer|2016|pp=221}} just with the elements of the pairs exchanged.

===Inversion number===

The '''inversion number''' <math>\mathtt{inv}(X)</math>{{sfn|Mannila|1985}} of a sequence <math>X=\langle x_1,\dots,x_n\rangle</math>, is the [[cardinality]] of the inversion set. It is a common measure of the sortedness (sometimes called presortedness) of a permutation{{sfn|Vitter|Flajolet|1990|pp=459}} or sequence.{{sfn|Barth|Mutzel|2004|pp=183}} The inversion number is between 0 and <math>\frac{n(n-1)}2</math> inclusive. A permutation and its inverse have the same inversion number.

For example <math>\mathtt{inv}(\langle1,2,\dots, n\rangle)=0</math> since the sequence is ordered. Also, when <math>n = 2m</math> is even, <math>\mathtt{inv}(\langle m+1,m+2,\dots,2m,1,2,\dots, m\rangle)=m^2</math> (because each pair <math>(1\le i\le m < j\le 2m)</math> is an inversion). This last example shows that a set that is intuitively "nearly sorted" can still have a quadratic number of inversions.

It is the number of crossings in the arrow diagram of the permutation,{{sfn|Gratzer|2016|pp=221}} its [[Kendall tau distance]] from the identity permutation, and the sum of each of the inversion related vectors defined below.

ItThe inversion number is the number of crossings in the arrow diagram of the permutation,{{sfn|Gratzer|2016|pp=221}} itsthe permutation's [[Kendall tau distance]] from the identity permutation, and the sum of each of the inversion related vectors defined below.

It does not matter if the place-based or the element-based definition of inversion is used to define the inversion number, because a permutation and its inverse have the same number of inversions.

Other measures of (pre-)sortedness include the minimum number of elements that can be deleted from the sequence to yield a fully sorted sequence, the number and lengths of sorted "runs" within the sequence, the Spearman footrule (sum of distances of each element from its sorted position), and the smallest number of exchanges needed to sort the sequence.{{sfn|Mahmoud|2000|pp=284}} Standard [[comparison sort]]ing algorithms can be adapted to compute the inversion number in time {{math|O(''n'' log ''n'')}}.{{sfn|Kleinberg|Tardos|2005|pp=225}}

===Inversion related vectors===

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[[File:Inversion example; Rothe 1.svg|thumb|Rothe diagram]]

'''Inversion vector <math>v</math>:'''<br>With the ''element-based'' definition <math>v(i)</math> is the number of inversions whose ''smaller'' (right) component is <math>i</math>.{{sfn|Knuth|1973|pp=11}}

With the ''element-based'' definition <math>v(i)</math> is the number of inversions whose ''smaller'' (right) component is <math>i</math>.{{sfn|Knuth|1973|pp=11}}

:<math>v(i)</math> is the number of elements in <math>\pi</math> greater than <math>i</math> before <math>i</math>.

:<math>v(i) ~~=~~ \# \{ k \mid k > i ~\land~ \pi^{-1}(k) < \pi^{-1}(i) \}</math>

'''Left inversion count <math>l</math>:'''<br>With the ''place-based'' definition <math>l(i)</math> is the number of inversions whose ''bigger'' (right) component is <math>i</math>.

With the ''place-based'' definition <math>l(i)</math> is the number of inversions whose ''bigger'' (right) component is <math>i</math>.

:<math>l(i)</math> is the number of elements in <math>\pi</math> greater than <math>\pi(i)</math> before <math>\pi(i)</math>.

:<math>l(i) ~~=~~ \# \left\{ k \mid k < i ~\land~ \pi(k) > \pi(i) \right\}</math>

'''Right inversion count <math>r</math>, often called ''[[Lehmer code]]'':'''<br>With the ''place-based'' definition <math>r(i)</math> is the number of inversions whose ''smaller'' (left) component is <math>i</math>.

With the ''place-based'' definition <math>r(i)</math> is the number of inversions whose ''smaller'' (left) component is <math>i</math>.

:<math>r(i)</math> is the number of elements in <math>\pi</math> smaller than <math>\pi(i)</math> after <math>\pi(i)</math>.

:<math>r(i) ~~=~~ \# \{ k \mid k > i ~\land~ \pi(k) < \pi(i) \}</math>

Both <math>v</math> and <math>r</math> can be found with the help of a '''[[Rothe diagram''']], which is a [[permutation matrix]] with the 1s represented by dots, and an inversion (often represented by a cross) in every position that has a dot to the right and below it. <math>r(i)</math> is the sum of inversions in row <math>i</math> of the Rothe diagram, while <math>v(i)</math> is the sum of inversions in column <math>i</math>. The permutation matrix of the inverse is the transpose, therefore <math>v</math> of a permutation is <math>r</math> of its inverse, and vice versa.

==Example: All permutations of four elements==

[[File:2-element subsets of 4 elements; array, hexagonal.svg|thumb|The six possible inversions of a 4-element permutation]]

The following sortable table shows the 24 permutations of four elements (in the <math>\pi</math> column) with their place-based inversion sets (in the p-b column), inversion related vectors (in the <math>v</math>, <math>l</math>, and <math>r</math> columns), and inversion numbers (in the # column). (The small columns with smaller print and no heading are reflections of the columns next to them, and can be used to sort them in [[colexicographic order]].)

It can be seen that <math>v</math> and <math>l</math> always have the same digits, and that <math>l</math> and <math>r</math> are both related to the place-based inversion set. The nontrivial elements of <math>l</math> are the sums of the descending diagonals of the shown triangle, and those of <math>r</math> are the sums of the ascending diagonals. (Pairs in descending diagonals have the right components 2, 3, 4 in common, while pairs in ascending diagonals have the left components 1, 2, 3 in common.)

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The set of permutations on ''n'' items can be given the structure of a [[partial order]], called the '''weak order of permutations''', which forms a [[lattice (order)|lattice]].

The [[Hasse diagram]] of the inversion sets ordered by the [[subset]] relation forms the [[skeleton (topology)|skeleton]] of a [[permutohedron]].

If a permutation is assigned to each inversion set using the place-based definition, the resulting order of permutations is that of the permutohedron, where an edge corresponds to the swapping of two elements with consecutive values. This is the weak order of permutations. The identity is its minimum, and the permutation formed by reversing the identity is its maximum.

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== See also ==

{{wikiversity|Inversion (discrete mathematics)}}

{{Commons category|Inversion (discrete mathematics)}}

* [[Factorial number system]]

* [[Permutation graph]]

* [[Permutation group#Transpositions, simple transpositions, inversions and sorting|Transpositions, simple transpositions, inversions and sorting]]

* [[Damerau–Levenshtein distance]]

* [[Parity of a permutation]]

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|url = https://archive.org/details/Comtet_Louis_-_Advanced_Coatorics

| chapter = 6.4 Inversions of a permutation of [n]

| publisher = D. Reidel Pub. Co | ___location = Dordrecht, Boston | year = 1974 | isbn = 9027704414 }}

* {{cite book

| first1=Thomas H. |last1=Cormen |authorlink1=Thomas H. Cormen

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|title=Algorithm Design

|year=2005

|publisher=Pearson/Addison-Wesley

|isbn=0-321-29535-8 }}

* {{cite book

| last1 = Knuth | first1 = Donald

| title = [[The artArt of computerComputer programmingProgramming]]

| chapter = 5.1.1 Inversions

| publisher = Addison-Wesley Pub. Co | year = 1973 | isbn = 0201896850}}

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=== Further reading ===

{{refbegin}}

* {{cite journal|journal=Journal of Integer Sequences|volume=4|year=2001|title=Permutations with Inversions|first=Barbara H.|last=Margolius|page=24|bibcode=2001JIntS...4...24M}}

{{refend}}

=== Presortedness measures ===

{{refbegin}}

*{{cite journal

* {{cite journal|journal=Lecture Notes in Computer Science|year=1984|volume=172|pages=324&ndash;336|doi=10.1007/3-540-13345-3_29|title=Measures of presortedness and optimal sorting algorithms|first=Heikki|last=Mannila|isbn=978-3-540-13345-2|authorlink=Heikki Mannila}}

| last = Mannila | first = Heikki | author-link = Heikki Mannila

| date = April 1985

| doi = 10.1109/tc.1985.5009382

| issue = 4

| journal = IEEE Transactions on Computers

| pages = 318–325

| title = Measures of presortedness and optimal sorting algorithms

| volume = C-34}}

* {{cite journal|first1=Vladimir|last1=Estivill-Castro|first2=Derick|last2=Wood|author2-link=Derick Wood|title=A new measure of presortedness|journal=Information and Computation|volume=83|issue=1|pages=111&ndash;119|year=1989|doi=10.1016/0890-5401(89)90050-3|doi-access=free}}

* {{cite journal|first=Steven S.|last=Skiena|year=1988|title=Encroaching lists as a measure of presortedness|journal=BIT|volume=28|issue=4|pages=755&ndash;784|doi=10.1007/bf01954897|s2cid=33967672}}