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different note, the lecture notes are unfortunately the best sources I could find, seems nobody published a paper about it or mentioned it in a book |
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Each node <code>v</code> is assigned a unique integer <code>v.index</code>, which numbers the nodes consecutively in the order in which they are discovered. It also maintains a value <code>v.lowlink</code> that represents the smallest index of any node on the stack known to be reachable from <code>v</code> through <code>v</code>'s DFS subtree, including <code>v</code> itself. Therefore <code>v</code> must be left on the stack if <code>v.lowlink < v.index</code>, whereas v must be removed as the root of a strongly connected component if <code>v.lowlink == v.index</code>. The value <code>v.lowlink</code> is computed during the depth-first search from <code>v</code>, as this finds the nodes that are reachable from <code>v</code>.
The lowlink is different from the lowpoint, which is the smallest index reachable from <code>v</code> through any part of the graph.<ref name=Tarjan/>{{rp|156}}<ref
== The algorithm in pseudocode ==
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When each node finishes recursing, if its lowlink is still set to its index, then it is the root node of a strongly connected component, formed by all of the nodes above it on the stack. The algorithm pops the stack up to and including the current node, and presents all of these nodes as a strongly connected component.
In Tarjan's paper, when <code>''w''</code> is on the stack, <code>''v''.lowlink</code> is updated with the assignment <code>''v''.lowlink := min(''v''.lowlink, ''w''.index)</code>.<ref name=Tarjan/>{{rp|157}} A common variation is to instead use <code>''v''.lowlink := min(''v''.lowlink, ''w''.lowlink)</code>.<ref>{{cite journal |last1=Kordy |first1=Piotr |last2=Langerak |first2=Rom |last3=Mauw |first3=Sjouke |last4=Polderman |first4=Jan Willem |title=A Symbolic Algorithm for the Analysis of Robust Timed Automata |journal=FM 2014: Formal Methods |date=2014 |volume=8442 |pages=351–366 |doi=10.1007/978-3-319-06410-9_25 |url=https://satoss.uni.lu/members/sjouke/papers/KLMP14.pdf}}</ref><ref>{{cite web |title=Lecture 19: Tarjan’s Algorithm for Identifying Strongly Connected Components in the Dependency Graph |url=http://courses.cms.caltech.edu/cs130/lectures/CS130-Wi2024-Lec19.pdf |website=CS130 Software Engineering |publisher=Caltech |date=Winter 2024}}</ref> This modified algorithm does not compute the lowlink numbers as Tarjan defined them, but the test <code>''v''.lowlink = ''v''.index</code> still identifies root nodes of strongly connected components, and therefore the overall algorithm remains valid.<ref name="CMU2018">{{cite web |title=Lecture #19: Depth First Search and Strong Components |url=https://www.cs.cmu.edu/~15451-f18/lectures/lec19-DFS-strong-components.pdf |website=15-451/651: Design & Analysis of Algorithms |publisher=Carnegie Mellon |date=1 November 2018}}</ref>
== Complexity ==
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