1. New Algorithms for Enumerating All Maximal Cliques
Kazuhisa Makino Takeaki Uno
Osaka University National Institute of
JAPAN Informatics, JAPAN
9/Jul/2004 SWAT 2004

2. Background ・ There are still many unsolved nice problems
Recently, Enumeration algorithms are interesting
・ There are still many unsolved nice problems
(unlike to ordinal discrete algorithms)
・ Recent increase of computer power makes
many enumeration problems practically solvable
 many applications have been appearing,
such as, genome, data mining, clustering, so on
・ Some (theoretical) algorithms use enumeration as subroutines
(recognition of perfect graph)

3. 5000 researchers use enumeration algorithms ?????
Background (cont.)
・ My institute has 100 researchers of informatics
・ At least 5 researchers (independently) use implementations of enumeration algorithms
・ Suppose that there are 100,000 researchers of informatics
in the world
5000 researchers use enumeration algorithms ?????

4. Problems and Results Problem1 : for a given graph G=(V, E),
enumerate all maximal cliques in G
Problem2 : for a given bipartite graph G=(V1∪V2, E),
enumerate all maximal bipartite cliques in G
( Problem2 is a special case of Problem1 )
・ We propose algorithms for solving these problems,
reduce the time complexity in dense cases and sparse cases.
・ Computational experiments for random graphs and real-world data

5. Difficulty ・ Consider branch-and-bound type enumeration:
divide maximal cliques into two groups
maximal cliques including v / not including v
・ If a group includes no maximal clique,  cut off the branch
 Finding a maximal clique not including given vertices of S
is NP-Complete
 Can not cut off subproblems(branches)
including no maximal clique
v1∈K
v1∈K
v2∈K
v2∈K

6. Existing Studies and Ours
O(|V||E|): Tsukiyama, Ide, Ariyoshi & Shirakawa,
O(|V||E|), lexicographic order: Johnson, Yanakakis & Papadimitriou
O(a(G)|E|): Chiba & Nishizeki
( a(G): arboricity of G with m/(n-1) ≦ a(G) ≦m1/2 )
・ many heuristic algorithms in data mining, for bipartite case
Ours:
O(|V|2.376) (dense case)
O(Δ4) (sparse case)
O((Δ*)4 + θ3 ) (θ vertices have degree > Δ* )
O(Δ3) (bipartite case)
O(Δ2) (bipartite case with using much memory)

7. Enumeration of Maximal Cliques
・ Improved version of algorithm of Tsukiyama et. al.
Idea: Construct a route on all maximal cliques to be traversed
・ For a maximal clique K of G = ( V, E ) :
C (K) : lexicographically maximum maximal clique including K
K≦i : vertices of K with indices ≦ i
i(K) : minimum index s.t. C(K≦i) = C(K≦i+1)
parent of a maximal clique K : C(K≦i(K)-1)
・ parent is lexicographically larger than K 3 4 6 7 9 9
Lexicographically
larger 4 1 11 7 8 10
1,2,3 > 1,2,4 3 10
1,3,6 > 1,4,5 2 K 6 8
i(K) 5

8. Graph Representation of Relation
・ Parent-child relation is acyclic
　  graph representation forms a tree (enumeration tree)
Visit all maximal cliques by depth-first search
・ need to find children of a maximal clique

9. Child of Maximal Clique
Γ(vi) : vertices adjacent to vi
K[i] = C ( K≦i ∩ Γ(vi) ∪ {vi} )
・ H is a child of K only if H = K[i] for some i>i(K)
(H is a child of K if the parent of K[i] is K )
・ i(K[i]) = i
・construct K[i] in O(|E|) time
・construct parent in O(|E|) time
( O(Δ2 ) time)
・for i=i(K)+1,…,|V| in O(|V||E|) time
 enumerate O(|V||E|) time
per maximal clique
K,i(K)=6 4 9 9 4 1 11 7
K[8] 10 8 3 10 2 6 8 5

10. Characterization of Child
The parent of K[i] = K ⇔
(1) no vj , j<i is adjacent to all vertices in K≦i ∩Γ(vi) ∪ {vi}
(2) no vj , j<i is adjacent to all vertices in K≦i∩Γ(vi) ∪ K≦j
(1) is not satisfied ⇔ K[i] and parent of K[i] includes vj∈K
(2) is not satisfied ⇔ parent of K[i] includes vj∈K 5
K = {3,4,7,9}
K[10] = {3,7,10}
K≦ = {3,4}
K ≦7∩Γ(v10) = {3,7} 7 1 4
K≦5∪ 4 9 3 10
K ≦10∩Γ(v10)
∪ {v10}

11. Use of Matrix Multiplication
・ Check the conditions (1) and (2) by matrix multiplication
(1) no vj , j<i is adjacent to all vertices in K ≦i ∩Γ(vi) ∪ {vi}
ith row of left ⇒ K≦i∩Γ(vi)∪{vi}
jth column of right ⇒ Γ(vj)
ij cell of product ⇒ | K≦i∩Γ(vi)∪{vi} ∩ Γ(vj) |
= |K≦i∩Γ(vi)∪{vi}| ?
Γ(vj) ∩
K ≦i ∩Γ(vi) ∪ {vi}
K≦i∩Γ(vi)∪{vi}
Γ(vj)
Condition (2) can be checked in the same way
Checked in O( |V|2.368 ) time ⇒ time complexity is O( |V|2.368 ) for each

12. Sparse Cases ・ If vi is adjacent to no vertex in K
 K[i] = C ( K≦i ∩ Γ(vi) ∪ {vi} ) = C ({vi})
 parent of K[i] = C ( C ({vi}) ≦i )
If C ({vi}) ≦i ＝φ, parent of K[i] is K0
If C ({vi}) ≦i ≠φ, (1) is not satisfied
 If K ≠ K0, K[i] is not a child of K
・ Since |K|≦Δ+1 , at most Δ(Δ+1) vertices are adjacent to K
・ Each K[i] takes O(Δ2) time to construct the parent
Δ: max. degree
O((Δ*)4 + |Θ|3 ) if partially dense
Δ*: max. degree in V＼Θ
O(Δ4 ) per maximal clique

13. Bipartite Clique
・ Enumerate maximal bipartite cliques in G =(V1 ∪V2 ,E )
( = maximal cliques in G’ = (V1 ∪V2 , E ∪V1 ×V1 ∪V2×V2 ))
 enumerated in O( |V|2.368 ) time for each
・ But a sparse bipartite graph will be dense
 need some improvements for sparse cases V1 V2

14. Fast Construction of K[i]
・ For any maximal bipartite clique K
K ∩V2 = ∩v∈K ∩V1 Γ(v)
K ∩V1 = ∩v∈K ∩V2 Γ(v)
・ K[i]∩V1 for all i are computed in O(Δ2) time
・ K[i] for all i are computed in O(Δ3) time
K[v1]
K[v6] v1 v2 v5 v6
Γ(1)
Γ(2)
Γ(3)
Γ(4)
K[i] V1 1 2 3 4 vi V2

15. Enumerated in O(Δ3) time for each
Checking the Parent
・ Put small indices to V1 , large indices to V2
 K[i] is a child of K ⇔ K[i]≦i = K≦i
 checked in O(Δ) time
・・・ V1 1 2 3
|V1|-1
|V1| V2
・・・
|V1|+1
|V1|+2
K[i] V1 vi V2
Enumerated in O(Δ3) time for each
O(Δ2) by using memory

16. Computational Experiments
・ for graphs randomly generated
・ vertex vi is connected to vertices from i-r to i+r with probability 1/2
・ Faster than Tsukiyama’s algorithm
・ Computation time is linear in maximum degree

17. Benchmark Problems
・ Problem of finding frequent closed item sets from database
 equivalent to maximal bipartite clique enumeration
・ Used on KDDcup (data mining algorithm competition )
BMS-WebView1　 (from Web-log data)
　　　 |V|= 60,000, ave. degree 2.5
BMS-WebView2　(from Web-log data)
　　　 |V|= 80,000, ave. degree 5
BMS-POS 　(from POS data)
　　　|V|= 510,000, ave. degree 6
IBM-Artificial　 (artificial data)
　　　|V|= 100,000 , ave.degree 10

18. Results

19. Conclusion and Future Work
・ Proposed fast algorithms for enumerating
maximal cliques: O(|V|2.376), O(Δ4 ), O((Δ*)4 + θ3 )
maximal bipartite cliques: O(|V|2.376), O(Δ3 ), O(Δ2)
・ Examined benchmark problems of data mining,
and showed that our algorithm performs well.
Future work:
・ Can we improve more? What is the difficulty ?
・ Can we enumerate other maximal (minimal) graph objects ?
・ Can we apply matrix multiplication to other enumeration problems ?
・ What can be enumerated efficiently in practice ?

20. Frequent Sets Input graph: An item and a customer is connected
iff the customer purchased the item
In a maximal bipartite clique:
Customers: have similar favorites
Items: frequently purchased together
[Agrawal et al. 96, Zaki et al. 02, Pei 00, Han 00, … ]
customer1
customer2
customer3
customer4
beer
nappy
milk

21. Few Large Degree Vertices
・ Very few vertices (denoted by Θ) have large degrees
・ Divide the maximal cliques into two groups:
(a) cliques not included in Θ
(b) cliques included in Θ
・ (a) can be enumerated in O(Δ’4) time
・ Maximal clique K in the induced graph by Θ is
a maximal clique of G ⇔ K is not included in any of (a)
 O(|Θ|3) time for each
small degree < Δ’
large
degree
O(Δ’4 + |Θ|3 ) per maximal clique

22. Avoid Duplications by Using Memory
・We can avoid duplications by storing all maximal bipartite cliques
・ From K ∩V1 =Γ(K ∩V2) , we store all K ∩V1
1. Get a K from memory (which is un-operated)
2. generate all K[i]∩V1
3. Store each K[i]∩V1 if it is not in memory
4. Go to 1 if a maximal clique is un-operated
Enumerated in O(Δ2) time for each