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{{Short description|Method for mathematical optimization}}
{{About|an algorithm for mathematical optimization||Criss-cross (disambiguation){{!}}Criss-cross}}
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[[File:Unitcube.svg|thumb|right|alt=A three-dimensional cube|The criss-cross algorithm visits all 8 corners of the [[Klee–Minty cube]] in the worst case. It visits 3 additional corners on average. The Klee–Minty cube is a perturbation of the cube shown here.]]
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The criss-cross algorithm is simpler than the simplex algorithm, because the criss-cross algorithm only has one phase. Its pivoting rules are similar to the [[Bland's rule|least-index pivoting rule of Bland]].<ref name="Bland">
{{cite journal|title=New finite pivoting rules for the simplex method|first=Robert G.|last=Bland|journal=Mathematics of Operations Research|volume=2|number=2|date=May 1977|pages=103–107|doi=10.1287/moor.2.2.103|jstor=3689647|mr=459599}}</ref> Bland's rule uses only [[sign function|sign]]s of coefficients rather than their [[real number#Axiomatic approach|(real-number) order]] when deciding eligible pivots. Bland's rule selects an entering variables by comparing values of reduced costs, using the real-number ordering of the eligible pivots.<ref name="Bland"/><ref>Bland's rule is also related to an earlier least-index rule, which was proposed by Katta G. Murty for the [[linear complementarity problem]], according to {{harvtxt|Fukuda|Namiki|1994}}.</ref> Unlike Bland's rule, the criss-cross algorithm is "purely combinatorial", selecting an entering variable and a leaving variable by considering only the signs of coefficients rather than their real-number ordering.<ref name="FukudaTerlaky"/><ref name="TerlakyZhang"/> The
While most simplex variants are monotonic in the objective (strictly in the non-degenerate case), most variants of the criss-cross algorithm lack a monotone merit function which can be a disadvantage in practice.
==Description==
The criss-cross algorithm works on a standard pivot tableau (or on-the-fly calculated parts of a tableau, if implemented like the revised simplex method). In a general step, if the tableau is primal or dual infeasible, it selects one of the infeasible rows / columns as the pivot row / column using an index selection rule. An important property is that the selection is made on the union of the infeasible indices and the standard version of the algorithm does not distinguish column and row indices (that is, the column indices basic in the rows). If a row is selected then the algorithm uses the index selection rule to identify a position to a dual type pivot, while if a column is selected then it uses the index selection rule to find a row position and carries out a primal type pivot.
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===Other optimization problems with linear constraints===
There are variants of the criss-cross algorithm for linear programming, for [[quadratic programming]], and for the [[linear complementarity problem|linear-complementarity problem]] with "sufficient matrices";<ref name="FukudaTerlaky"/><ref name="FTNamiki"/><ref name="FukudaNamikiLCP" >{{harvtxt|Fukuda|Namiki|1994|}}</ref><ref name="OMBook" >{{cite book|last1=Björner|first1=Anders|last2=Las Vergnas|first2=Michel|author2-link=Michel Las Vergnas|last3=Sturmfels|first3=Bernd|author-link3=Bernd Sturmfels|last4=White|first4=Neil|last5=Ziegler|first5=Günter|author-link5=Günter M. Ziegler|title=Oriented Matroids|chapter=10 Linear programming|publisher=Cambridge University Press|year=1999|isbn=978-0-521-77750-6|pages=417–479|doi=10.1017/CBO9780511586507|mr=1744046}}</ref><ref name="HRT">{{cite journal|first1=D. |last1=den Hertog|first2=C.|last2=Roos|first3=T.|last3=Terlaky|title=The linear complementarity problem, sufficient matrices, and the criss-cross method|journal=Linear Algebra and Its Applications|volume=187|date=1 July 1993|pages=1–14|url=
===Vertex enumeration===
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==Notes==
{{Reflist}}
==References==
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* {{cite journal|last1=Fukuda|first1=Komei|author-link1=Komei Fukuda|last2=Namiki|first2=Makoto|title=On extremal behaviors of Murty's least index method|journal=Mathematical Programming|date=March 1994|pages=365–370|volume=64|number=1|doi=10.1007/BF01582581|mr=1286455|s2cid=21476636}}
* {{cite journal|first1=Komei|last1=Fukuda| author-link1=Komei Fukuda |first2=Tamás|last2=Terlaky| author-link2=Tamás Terlaky |title=Criss-cross methods: A fresh view on pivot algorithms |journal=Mathematical Programming, Series B|volume=79|pages=369–395|issue=Papers from the 16th International Symposium on Mathematical Programming held in Lausanne, 1997, number 1–3 |editor1-first=Thomas M.|editor1-last=Liebling|editor2-first=Dominique|editor2-last=de Werra|year=1997|doi=10.1007/BF02614325|mr=1464775|id=[http://www.cas.mcmaster.ca/~terlaky/files/crisscross.ps Postscript preprint]|citeseerx=10.1.1.36.9373|s2cid=2794181}}
* {{cite journal|first1=D.|last1=den Hertog|first2=C.|last2=Roos|first3=T.|last3=Terlaky|title=The linear complementarity problem, sufficient matrices, and the criss-cross method|journal=Linear Algebra and Its Applications|volume=187|date=1 July 1993|pages=1–14|url=
* {{<!-- citation -->cite journal|title=The finite criss-cross method for hyperbolic programming|journal=European Journal of Operational Research|volume=114|number=1|
pages=198–214|year=1999<!-- |issn=0377-2217 -->|doi=10.1016/S0377-2217(98)00049-6|url=http://www.sciencedirect.com/science/article/B6VCT-3W3DFHB-M/2/4b0e2fcfc2a71e8c14c61640b32e805a
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