Archive for the ‘Equivalence Class’ Category

Succinct data structures for representing equivalence classes

Monday, June 24th, 2013

Succinct data structures for representing equivalence classes by Moshe Lewenstein, J. Ian Munro, and Venkatesh Raman.

Abstract:

Given a partition of an n element set into equivalence classes, we consider time-space tradeoffs for representing it to support the query that asks whether two given elements are in the same equivalence class. This has various applications including for testing whether two vertices are in the same component in an undirected graph or in the same strongly connected component in a directed graph.

We consider the problem in several models.

— Concerning labeling schemes where we assign labels to elements and the query is to be answered just by examining the labels of the queried elements (without any extra space): if each vertex is required to have a unique label, then we show that a label space of (\sum_{i=1}^n \lfloor {n \over i} \rfloor) is necessary and sufficient. In other words, \lg n + \lg \lg n + O(1) bits of space are necessary and sufficient for representing each of the labels. This slightly strengthens the known lower bound and is in contrast to the known necessary and sufficient bound of \lceil \lg n \rceil for the label length, if each vertex need not get a unique label.

–Concerning succinct data structures for the problem when the n elements are to be uniquely assigned labels from label set {1, 2, …n}, we first show that \Theta(\sqrt n) bits are necessary and sufficient to represent the equivalence class information. This space includes the space for implicitly encoding the vertex labels. We can support the query in such a structure in O(\lg n) time in the standard word RAM model. We then develop structures resulting in one where the queries can be supported in constant time using O({\sqrt n} \lg n) bits of space. We also develop space efficient structures where union operation along with the equivalence query can be answered fast.

On the down side, this technique would not support merging based on arbitrary choice of properties.

On the up side, this technique does support merging based on pre-determined properties for merging.

The latter being the more common case, I commend this article to you for a close read.

Parametric matroid of rough set

Friday, September 28th, 2012

Parametric matroid of rough set by Yanfang Liu, William Zhu. (ArXiv.org for the first author, DBLP for the second.)

Abstract:

Rough set is mainly concerned with the approximations of objects through an equivalence relation on a universe. Matroid is a combinatorial generalization of linear independence in vector spaces. In this paper, we define a parametric set family, with any subset of a universe as its parameter, to connect rough sets and matroids. On the one hand, for a universe and an equivalence relation on the universe, a parametric set family is defined through the lower approximation operator. This parametric set family is proved to satisfy the independent set axiom of matroids, therefore it can generate a matroid, called a parametric matroid of the rough set. Three equivalent representations of the parametric set family are obtained. Moreover, the parametric matroid of the rough set is proved to be the direct sum of a partition-circuit matroid and a free matroid. On the other hand, since partition-circuit matroids were well studied through the lower approximation number, we use it to investigate the parametric matroid of the rough set. Several characteristics of the parametric matroid of the rough set, such as independent sets, bases, circuits, the rank function and the closure operator, are expressed by the lower approximation number.

If you are guessing this isn’t the “simpler” side of topic maps, you are right in one!

There are consumers of information/services (herein of “simpler” services of topic maps), authors of information/services (herein of semantic diversity by whatever tools), and finally, semantic intermediaries, map makers that cross the boundaries of semantic universes of discourse (here be dragons).

Not every aspect of topic maps is for everyone and we should not pretend otherwise.

Concord: A Tool That Automates the Construction of Record Linkage Systems

Sunday, November 27th, 2011

Concord: A Tool That Automates the Construction of Record Linkage Systems by Christopher Dozier, Hugo Molina Salgado, Merine Thomas, Sriharsha Veeramachaneni, 2010.

From the webpage:

Concord is a system provided by Thomson Reuters R&D to enable the rapid creation of record resolution systems (RRS). Concord allows software developers to interactively configure a RRS by specifying match feature functions, master record retrieval blocking functions, and unsupervised machine learning methods tuned to a specific resolution problem. Based on a developer’s configuration process, the Concord system creates a Java based RRS that generates training data, learns a matching model and resolves record information contained in files of the same types used for training and configuration.

A nice way to start off the week! Deeply interesting paper and a new name for record linkage.

Several features of Concord that merit your attention (among many):

A choice of basic comparison operations with the ability to extend seems like a good design to me. No sense overwhelming users with all the general comparison operators, to say nothing of the domain specific ones.

The blocking functions, which operate just as you suspect, narrows the potential set of records for matching down, is also appealing. Sometimes you may be better at saying what doesn’t match than what does. This gives you two bites at a successful match.

Surrogate learning, although I have located the paper cited on this subject and will be covering it in another post.

I have written to ThomsonReuters inquiring about availability of Concord, its ability to interchange mapping settings between instances of Concord or beyond. Will update when I hear back from them.

NAQ Tree in Your Forest?

Tuesday, January 25th, 2011

Effectiveness of NAQ-tree as index structure for similarity search in high-dimensional metric space Authors: Ming Zhang and Reda Alhajj Keywords: Knn search, High dimensionality, Dimensionality reduction, Indexing, Similarity search

Abstract:

Similarity search (e.g., k-nearest neighbor search) in high-dimensional metric space is the key operation in many applications, such as multimedia databases, image retrieval and object recognition, among others. The high dimensionality and the huge size of the data set require an index structure to facilitate the search. State-of-the-art index structures are built by partitioning the data set based on distances to certain reference point(s). Using the index, search is confined to a small number of partitions. However, these methods either ignore the property of the data distribution (e.g., VP-tree and its variants) or produce non-disjoint partitions (e.g., M-tree and its variants, DBM-tree); these greatly affect the search efficiency. In this paper, we study the effectiveness of a new index structure, called Nested-Approximate-eQuivalence-class tree (NAQ-tree), which overcomes the above disadvantages. NAQ-tree is constructed by recursively dividing the data set into nested approximate equivalence classes. The conducted analysis and the reported comparative test results demonstrate the effectiveness of NAQ-tree in significantly improving the search efficiency.

Although I think the following paragraph from the paper is more interesting:

Consider a set of objects O = {o1 , o2 , . . . , on } and a set of attributes A = {a1 , a2 , . . . , ad }, we first divide the objects into groups based on the first attribute a1 , i.e., objects with same value of a1 are put in the same group; each group is an equivalence class [23] with respect to a1 . In other words, all objects in a group are indistinguishable by attribute a1 . We can refine the equivalence classes further by dividing each existing equivalence class into groups based on the second attribute a2 ; all objects in a refined equivalence class are indistinguishable by attributes a1 and a2 . This process may be repeated by adding one more attribute at a time until all the attributes are considered. Finally, we get a hierarchical set of equivalence classes, i.e., a hierarchical partitioning of the objects. This is roughly the basic idea of NAQ-tree, i.e., to partition the data space in our similarity search method. In other words, given a query object o, we can gradually reduce the search space by gradually considering the most relevant attributes.

With the caveat that this technique is focused on metric spaces.

But I rather like the idea of reducing the search space by the attributes under consideration. Replace search space with similarity/sameness space and you will see what I mean. Still relevant for searching as well.