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# Background on random projections in euclidean spaces

Dimensionality reduction may be obtained by mapping points from a high to a low-dimensional space, approximately preserving some characteristics, i.e. the distances between points. In this context randomized embeddings with low distortion represent a key concept. Randomized embeddings have been successfully applied both to combinatorial optimization and data compression [12].

A randomized embedding between normed metric spaces with distortion , with and failure probability is a distribution probability over mappings , such that for every pair , the following property holds with probability :

 (1)

The main result on randomized embedding is due to Johnson and Lindenstrauss [13], who proved the existence of a randomized embedding with distortion and failure probability , for every . As a consequence, for a fixed data set , with , by union bound, for all , it holds:
 (2)

Hence, by choosing such that , it is proved the following:
Johnson-Lindenstrauss (JL) lemma: Given a set with there exists a -distortion embedding into with , where is a suitable constant.

The embedding exhibited in [13] consists in random projections from into , represented by matrices with random orthonormal vectors. Similar results may be obtained by using simpler embeddings, represented through random matrices , where are random variables such that:

For sake of simplicity, we call random projections even this kind of embeddings.

We may build random matrices in different ways to obtain random projections obeying the Johnson-Lindenstrauss lemma:

• The Plus-Minus-One (PMO) projection: , randomly chosen such that:

• The Achlioptas projection [1]:

• Generalized Achlioptas projection:

with .

• A Normal projection: .

• Other projections such that:

A particular case of randomized map is represented by Random Subspace (RS) projections [11]. Indeed these projections do not satisfy the JL lemma and may induce large distortions into the projected data. They are represented by matrices , where are uniformly chosen with entries in , and with exactly one per row and at most one per column. It is worth noting that, in this case, the "compressed" data set can be quickly computed in time , independently from . Unfortunately, it easy to see that in this case and and hence RS does not satisfy the JL lemma.

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Giorgio 2006-08-16