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DISCOVERING MOTIFS IN TIME SERIES. Duong Tuan Anh Faculty of Computer Science and Technology Ho Chi Minh City University of Technology. Tutorial MIWAI December 2012. OUTLINE. Introduction Definitions of time series motifs Applications of time series motifs

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Discovering motifs in time series

DISCOVERING MOTIFS IN TIME SERIES

Duong Tuan Anh

Faculty of Computer Science and Technology

Ho Chi Minh City University of Technology

Tutorial MIWAI December 2012


Outline

OUTLINE

  • Introduction

  • Definitions of time series motifs

  • Applications of time series motifs

  • Some well-known algorithms of finding motifs

  • Our proposed method

  • Conclusions


Introduction

INTRODUCTION


What are time series

What are time series?

A time series is a collection of observations made sequentially in time

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Examples: Financial time series, scientific time series


Time series data mining

Time series data mining

Q. Yang & X. Wu, “10 Challenging Problems in Data Mining Research”, Int. Journal on Information Technology and Decision Making, Vol. 5, No. 4 (2006), 597-604

3.Mining sequence data and time series data


Time series data mining1

Time series data mining

Time series data mining is a field of data mining to deal with the challenges from the characteristics of time series data.

Time series data have the following characteristics:

Very large datasets (terabyte-sized)

Subjectivity (The definition of similarity depends on the user)

Different sampling rates

Noise, missing data, etc.


What do we want to do with the time series data

What do we want to do with the time series data?

Classification

Clustering

Query by Content

Rule Discovery

Motif Discovery

10

s = 0.5

c = 0.3

Visualization

Novelty Detection


Time series motifs

Time series motifs

  • Motif: the most frequently occurring pattern in a longer time series


Motif discovery

Motif Discovery

Problem Description

Unsupervised detection andmodeling of previously unknownrecurring patterns in real-valued

time series

Discovery due to unknowns

  • Number of motifs + occurrences

  • Location and length of occurrences

  • “Shape” of each motif


Definitions

DEFINITIONS

J. Lin, E. Keogh, Patel, P. and Lonardi, S., Finding Motifs in Time Series, The 2nd Workshop on Temporal Data Mining, at the 8th ACM SIGKDD Int. Conf. on Knowledge Discovery and Data Mining, Edmonton, Alberta, Canada, 2002.


Definitions1

Definitions

  • Definition 1 Time Series:

    • A time series T = t1,…,tm is an ordered set of m real-valued variables.

  • Definition 2 Subsequences:

    • Given a time series T of length m, a subsequence C of T is a sampling of length n ≤ m of contiguous position from T, that is, C = tp,…,tp+n-1 for 1≤ p ≤ m – n + 1.

  • Definition 3. Match:

    • Given a positive real number R (called range) and a time series T containing a subsequence C beginning at position p and a subsequence M beginning at q, if D(C, M) ≤ R, then M is called a matching subsequence of C.


Definitions2

Definitions

  • Definition 4 Trivial Match:

    • Given a time series T, containing a subsequence C beginning at position p and a matching subsequence M beginning at q, we say that M is a trivial match to C if either p = q or there does not exist a subsequence M’ beginning at q’ such that D(C, M’) > R, and either q < q’< p or p < q’< q.


Definitions3

Definitions

  • Definition 5 1-Motif:

    Given a time series T, a subsequence of length n and a range R, the most significant motif in T (called 1-Motif) is the subsequence C1 that has the highest count of non-trivial matches.

  • Definition 6K-Motifs:

    The K-th most significant motif in T (called thereafter K-Motif) is the subsequence CK that has the highest count of non-trivial matches, and satisfies D(CK, Ci) > 2R, for all 1 ≤ i < K .


Definitions4

Definitions

  • K-Motifs:

If the motifs are only required to be R distance apart as in A, then the two motifs may share the majority of their elements. In contrast, B illustrates that requiring the centers to be at least 2R apart insures that the motifs are unique.


Algorithm find 1 motif brute force t n r

Algorithm Find-1-Motif-Brute-Force(T, n, R)

best_motif_count_so_far = 0

best_motif_location_so_far = null;

fori = 1 to length(T) – n + 1

count = 0; pointers = null;

forj = 1 to length(T) – n + 1

if Non_Trivial_Match (C[i: i + n – 1], C[j: j + n – 1], R) then

count = count + 1;

pointers = append (pointers, j);

end

end

if count > best_motif_count_so_far then

best_motif_count_so_far = count;

best_motif_location_so_far = i;

motif_matches = pointers;

end

end

The algorithm requires O(m2) calls to the distance function.

This procedure calls

distance function


Some applications of time series motifs

SOME APPLICATIONS OF TIME SERIES MOTIFS


Motif based classification of time series buza et al 2009

Motif-based classification of time series(Buza et al.,2009)

Motifs can be used for time series classification. This can be done in two steps:

  • (i) Motifs of all time series are extracted

  • (ii) Each time series is represented as an attribute vector using motifs so that a classifier like SVM, Naïve Bayes, etc. can be applied.

Buza, K. and Thieme, L. S.: Motif-based Classification of Time Series with Bayesian Networks and SVMs. In: A. Fink et al. (eds.) Advances in Data Analysis, Data Handling and Business Intelligences, Studies in Classification, Data Analysis, Knowledge Organization. Springer-Verlag, pp. 105-114 (2010).


Motif based clustering of time series phu anh 2011

Motif-based clustering of time series(Phu & Anh, 2011)

Motif information are used to initialization k-means clustering of time series:

  • Step 1: We find 1-motifs for all time series in the database.

  • Step 2: We apply k-Means clustering on the 1-motifs of all time series to obtain the clusters of motifs. From the centers of the motif clusters, we derive the associated time series and choose these time series as initial centers for the k-Means clustering .

Phu, L. and Anh, D. T., Motif-based Method for Initialization k-Means Clustering of Time Series Data, Proc. of 24th Australasian Joint Conference (AI 2011), Perth, Australia, Dec. 5-8. Dianhui Wang, Mark Reynolds (Eds.), LNAI 7106, Springer-Verlag, 2011, pp. 11-20.


Discovering motifs in time series

Motif-based time series predictionStock Temporal Prediction based on Time Series Motifs (Jiang et al., 2009)

  • For a certain length n, we can find a motif. A motif is a set of subsequences that are non-trivial matches with each other. Each subsequence in this set is called an instance of the motif

  • For different lengths of subsequences, we can find different motifs from a time series.

  • The idea: If the subsequence in the current sliding window matches with the prefix of a particular motif, we can predict that it will go like the suffix of the motif.


Motif based time series prediction cont

Motif-based time series prediction (cont.)

  • But the subsequence in the current window may be fit for a number of motifs and it makes many possibilities of the suffix.

  • Rule: If a subsequence is similar with every instance in a motif, then we can conclude that it belongs to the motif and we can use the motif for prediction.

  • This method is applied for short-term stock prediction

Jiang, Y., Li, C., Han, J.: Stock temporal prediction based on time series motifs. In: Proc. of 8th Int. Conf. on Machine Learning and Cybernetics, Baoding, China, July 12-15 (2009).


Signature verification using time series motifs gruber et al 2006

Signature verification using time series motifs (Gruber et al., 2006)

The process consists of 4 steps:

  • Step 1: Signatures are converted to time series

  • Step 2: Time series motifs are extracted using EP_C algorithm (using important Extreme points and Clustering)

  • Step 3: Motifs are used to train a dynamic radian basis function network (DRBF) that can classify time series

  • Step 4: Time series classification is applied to online signature verification

Gruber C., Coduro, M., Sick, B.: Signature Verification with Dynamic RBF Networks and Time Series Motifs. In : Proc of 10th Int. Workshop on Frontiers in Handwriting Recognition (2006).


Finding repeated images in database of shapes xi et al 2007

Finding repeated images in database of shapes (Xi et al., 2007)

  • Convert 2-dimensional shapes into time series

  • Find repeated images or “image motifs” in these time series

  • Define a new form of Euclidean distance ( “Rotation invariant Euclidean distance”)

  • Use a modified variant of Random projection

  • “Image motifs” can be applied in anthropology, palaeography (study of old texts) and zoology.

Xi, X., Keogh, E., Wei, L., Mafra-Neto, A., Finding Motifs in a Database of Shapes, Proc. of SIAM 2007, pp. 249-270.


Random projection mueen keogh algorithm

Random Projection

Mueen-Keogh Algorithm

TWO WELL-KNOWN ALGORITHMS OF FINDING TIME SERIES MOTIF


1 random projection b chiu 2003

1. Random projection (B. Chiu, 2003)

  • Algorithm adapting PROJECTION method for detecting motifs in biological sequences to detecting time series motifs.

  • It’s based on locality-preserving hashing.

  • Algorithm requires some pre-processing:

    • First, apply PAA , a method for dimensionality reduction

    • Discretize the transformed time series into symbolic strings (apply SAX)


Paa for dimensionality reduction

PAA for dimensionality reduction

  • Time series databases are often extremely large. Searching directly on these data will be very complex and inefficient.

  • To overcome this problem, we should use some of transformation methods to reduce the magnitude of time series.

  • These transformation methods are called dimensionality reduction techniques.

  • Some popular dimensionality reductions: DFT (Discrete Fourier Transform), DWT (Discrete Wavelet Transform), PAA (Piecewise aggregate Approximation), etc.


Discovering motifs in time series

PAA

  • To reduce the time series from n dimensions to w dimensions, the data is divided into w equal-sized segments.

  • The mean value of the data within a segment is calculated and a vector of these values becomes the data-reduced representation.


Discretization with sax

DISCRETIZATION WITH SAX

  • Discretization of a time series is tranforming it into a symbolic string.

  • The main benefit of this discretization: there is an enormous wealth of existing algorithms and data structures that allow the efficient manipulations of symbolic representations.

  • Lin and Keogh et al. (2003) proposed a method called Symbolic Aggregate Approximation (SAX), which allows the descretization of original time series into symbolic strings.

  • This discretization method is based on PAA representation and assumes normality of the resulting aggregated values.

  • SAX is a process which maps the PAA representation of the time series into a sequence of discrete symbols.


Discovering motifs in time series

SAX

  • Let a be the size of the alphabet that is used to discretize the time series. To symbolize the time series we have to find the values:

    where

    B = 1,…,a-1 are called breakpoints (0 and a are defined as - and +).

  • We notice that real financial time series often have a Gaussian distribution. To expect the equal likelihood (1/a) for each symbol, we have to pick the values basing on a statistical table for Gaussian distribution.

  • Definition: Breakpoints are a sorted list of number B = 1,…,a-1 such that the area under a Gausian curve from i to i+1 = 1/a.


Breakpoints and symbols

Breakpoints and symbols

Using the breakpoints, the time series

will be discretized into the symbolic string C = c1c2….cw. Each segment will be coded as a symbol ciusing the formula:

where k indicates the k-th symbol in the alphabet, 1 the 1st symbol in the alphabet and a the a-th symbol in the alphabet.


Discovering motifs in time series

  • For example, Table 1 gives the breakpoints for the values of a from 3 to 10.

  • Assume the size of the alphabet is a = 3, we divide the range of time series values into three segments in such a way that the accumulative probability distribution of each segment is equal (1/3).

  • Based on the standard normal distribution, 1 corresponds to  value when P( > x) = 1/3; 2 corresponds to  value when P( > x) = 1/3 + 1/3 = 2/3. Therefore, we get 1 = -0.43, 2 = 0.43 and these two breakpoints correspond to P(1 > x) = 0.33 and P(2 > x) = 0.66, respectively.


Discovering motifs in time series

Table 1: A lookup table that contains the breakpoints that divide a Gaussian distribution in an arbitrary number (from 3 to 10) of equiprobable regions.


Note we made two parameter choices

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Random projection algorithm

Random projection algorithm

  • It uses a collision matrix whose rows and columns are SAX representation of each time series subsequence.

  • At each iteration, it selects certain positions of each words (as a “mask”) and traverses the word list.

  • For each match, the collision matrix entry is incremented.

  • At the end, the large entries in the collision matrix are selected as motif candidates. (greater than a threshold s)

  • Finally, each candidate is checked for validity in the original data.


Random projection cont

Random projection (cont.)


A working example

A working example


A working example1

A working example


Remarks on random projection

Remarks on Random Projection

  • It’s the most popular algorithm for detecting time series motifs.

  • It can find motifs in linear time and is robust to noise.

  • However, it still has three drawbacks.

    • (i) it has several parameters that need to be tuned by user.

    • (ii) if the distribution of the projections is not sufficiently wide, it becomes quadratic in time and space.

    • (iii) it is based on locality-preserving hashing that is effective for a relative small number of projected dimensions (10 to 20).

  • And it’s quite slow for large time series.


2 mueen keogh algorithm mueen and keogh 2009

2. Mueen-Keogh Algorithm(Mueen and Keogh, 2009)

  • Based on the Brute-force algorithm

  • MK works directly on raw time series data

  • Three techniques to speed up the algorithm:

    • Exploiting the Symmetry of Euclidean Distance

    • Exploiting Triangular Inequality and Reference Point

    • Applying Early Abandoning


Exploiting the symmetry of euclidean distance

Exploiting the Symmetry of Euclidean Distance

  • Basing on D(A, B) = D(B, A), we can prune off a half of the distance computations by storing D(A, B) and reusing the value when we need to find D(B, A).


Exploiting triangular inequality and reference point

Exploiting Triangular Inequality and Reference Point

  • Given two subsequences Ca and Cb. By triangular inequality, we have

    D(Q, Ca) D(Q, Cb) + D(Ca, Cb).

  • From that, we derive: D(Ca, Cb) D(Q, Ca) – D(Q, Cb).

  • If we want to check whether D(Ca, Cb)R , we only need to look at D(Q, Ca) – D(Q, Cb). If D(Q, Ca) – D(Q, Cb)R, we can conclude that D(Ca, Cb)R.

  • Given a reference subsequenceQ, we have to compute the distances from Q to all the subsequences in time series Ti. That means we have to compute D(Q, ti) for each subsequence ti in the time series Ti.


Applying early abandoning

Applying Early Abandoning

  • In the case the triangular inequality can not help, we have to compute the Euclidean distance D(Ca, Cb), then we can apply early abandoning technique.

  • The idea: we can abandon the Euclidean distance computation as soon as the cumulative sum during distance computation goes beyond the range R.


Experiments of mk algorithm

Experiments of MK Algorithm

  • Limitation: The use of Euclidean distance directly on raw time series data gives rise to robustness problem when dealing with noisy data.

Table 1: Experiments on the number of distance function calls (Stock dataset)


Our proposed method ep c algorithm

Our proposed method: EP-C algorithm


Significant extreme points clustering gruber et al 2006

Significant Extreme Points & Clustering(Gruber et al., 2006)

  • We can compress a time series by selecting some of its minima and maxima, called important points and dropping the other points.


Important extreme points

Important extreme points

  • Important minimum:

    • am T= {a1,…, an} is an important minimum if there are i, j where i < m < j, such that:

      • am is the minimum among ai, …, aj, and

      • ai/am ≥ R and aj/am ≥ R (R is the compression rate)

  • Important maximum:

    • am T= {a1,…, an} is an important maximum if there are i, j where i< m < j, such that:

      • am is the maximum among ai, …, aj, and

      • am/ai ≥ R and am/aj≥ R (R is the compression rate)


Finding time series motifs

Finding Time Series Motifs

  • (i) Compute all important extreme points

  • (ii) Extract candidate motifs

  • (iii) Clustering of candidate motifs

  • (iv) Select the motifs from the result of the clustering

K. B. Pratt and E. Fink, “Search for patterns in compressed time series”, International Journal of Image and Graphics, vol. 2, no. 1, pp. 89-106, 2002.


Extracting motif candidates

Extracting Motif Candidates

Function getMotifCandidateSequence(T)

N = length(T);

EP = findSignificantExtremePoints(T, R);

maxLength = MAX_MOTIF_LENGTH;

for i = 1 to (length(EP)-2) do

motifCandidate = getSubsequence(T, epi, epi+2)

if length(motifCandidate) > maxLength

then

addMotifCandidate(resample(motifCandidate, maxLength))

else

addMotifCandidate(motifCandidate)

end if

end for

end

Spline Interpolation or homothety


Homothetic transformation

Homothetic transformation

Homothetyis a transformation in affine space. Given a point O and a valuek ≠ 0. A homothety with center O and ratio k transforms M to M’ such that .

The Figure shows a homothety with center O and ratio k = ½ which transforms the triangle MNP to the triangle M’N’P’.


Homothety cont

Homothety (cont.)

The algorithm that performs homothety to transform a motif candidate T with length N (T = {Y1,…,YN}) to motif candidate of length N’ is given as follows.

  • 1. Let Y_Max = Max{Y1,…,YN}; Y_Min = Min {Y1,…,YN}

  • 2. Find a center I of the homothety with the coordinate: X_Center = N/2, Y_Center = (Y_Max + Y_Min)/2

  • 3. Perform the homothety with center I and ratio k = N’/N.


Minimum euclidean distance

Minimum Euclidean Distance

  • Besides, we note that two ‘similar’ motif candidates will not be recognized where a vertical difference exists between them.

  • To make the clustering step in the EP-C algorithm able to handle not only uniform scaling but also shifting transformation along vertical axis, we modify our Euclidean distance by using Minimum Euclidean Distance as a method of negating the differences caused through vertical axis offsets.

  • Given two motif candidates: T’ = {T’1, T’2,…,T’N} and Q’ = {Q’1, Q’2,…,Q’N}, the Euclidean distance between them is normally given by the following formula:


Discovering motifs in time series

a) Uniform scaling

b) Shifting along the vertical axis


Minimum euclidean distance1

Minimum Euclidean Distance

  • In this work we use the Minimum Euclidean Distance which is defined by the following equation:

where b (2)

From Eq. (2), we can derive a suitable value for the shifting parameter b such that we can find the best match between the two motif candidates Q’ and T’ as follows:


Clustering of motif candidates

Clustering of Motif Candidates

function getHierarchicalClustering(MCS, u, d)

C = getIntitialClustering(MCS)

while size(C)>u do

[Ci, Cj] = getMostSimilarClusters()

addCluster(C, mergeClusters(Ci, Cj )

removeCluster(C, Ci);

removeCluster(C, Cj );

endwhile

return C

end


Comparing the ep c algorithm to random projection

Comparing the EP-C algorithm to Random Projection

  • For EP-C, we implement three variants of EP-C in order to compare EP-C using spline interpolation (for resampling a longer motif candidate to a shorter one) to EP-C using homothety and compare EP-C using HAC clustering to EP-C using K-means clustering. We denote these variants as follows:

    • HAC|SI: the EP-C algorithm using spline interpolation and HAC clustering.

    • HAC|HT: the EP-C algorithm using homothety and HAC clustering.

    • K-means|HT: the EP-C algorithm using homothety and K-means clustering.


The test datasets

The test datasets

  • In this experiment, we tested the algorithms on four publicly available datasets:

    • ECG (7900 data points),

    • Memory (6800 data points),

    • Power (35000 data points) and

    • ECG (140000 data points).

  • All these four datasets are obtained from the UCR Time Series Data Mining Archive (Keogh and Folias, 2002).


Parameter settings

Parameter settings

  • For the RP algorithm, we use the parameter setting: the length of each PAA segment l = 16, the alphabet size a = 5, the number of iterations in RP i = 10, the number of errors allowed by RP d = 1. The SAX word length w depends on the dataset, for example, w = 15 for ECG dataset and w = 26 for Memory dataset.

  • For the EP-C algorithm, we use the parameter setting: the compression ratio for extracting significant extreme points R = 1.2, the minimum length of motif candidates l_min = 50 and r = the number of clusters / the number of extreme points = 0.2. When HAC clustering is used, the distance between two clusters is computed by the minimum distance, i.e. the distance between two nearest objects each belonging to one of the two clusters.


Experimental results

Experimental results


Discovering motifs in time series

From the experimental results, we can see that:

  • 1. The three variants of EP-C algorithm are much more effective than RP in terms of time efficiency and accuracy. The number of motif instances in the case of RP is more than that of EP-C. The reason of this fact is that RP uses sliding window to extract subsequences for motif discovery while EP-C uses the concept of significant extreme points to extract motif candidates. However, the accuracy of EP-C is still better than that of RP.

  • 2. With large datasets such as Power (35000 data points) and ECG (140000 data points), RP can not work since it can not tackle large datasets, while HAC|HT can find the motif in a very short time (2 or 3 seconds).

  • 3. The EP-C with homothety (HAC|HT) is better than EP-C with spline interpolation (HAC|SI) in terms of time efficiency and accuracy.

  • 4. HAC clustering is more suitable than K-means for clustering motif candidates in the EP-C algorithm.


Discovering motifs in time series

Left) ECG dataset. Right) A motif was discovered in the dataset.

Left) Power dataset. Right) A motif was discovered in the dataset.


Efficiency of ep c

Efficiency of EP-C

  • We can evaluate the efficiency of the EP-C algorithm by simply considering the ratio of how many times the Euclidean distance function must be called by EP-C over the number of times it must be called by the brute-force algorithm given in (Lin et al., 2002).

  • Note: the numbers of Euclidean distance function calls in the EP-C are the same for all three different versions of EP-C (HAC|SI, HAC|HT and k-Means|HT).

  • Table 2: The efficiency of the EP-C algorithm on various datasets

    Dataset ECG Memory Power ECG

    7900 6800 35000 140000

    ----------------------------------------------------------------------------

    Efficiency 0.0000638 0.0000920 0.0001304 0.0000635


Remarks on ep c algorithm

Remarks on EP_C Algorithm

This motif discovery method (EP_C) is very efficient, especially for large time series. It is much more efficient than Random Projection.

For example, experiment on Koski_ECG dataset from UCR Archive: http://www.cs.ucr.edu/~eamonn/SAX/koski_ecg.dat

This time series has 144002 points, run time for detection motif: 3secs


Conclusions

Conclusions

  • Time series motif discovery has several practical applications.

  • Through experiments, we see that EP_C (Extreme points and Clustering) method are much more efficient than Random Projection.


Future works

Future works

  • Developing methods to detect motifs in streaming time series.

  • Exploiting motif information in time series prediction:

    • Banking data

    • Environmental data


Thank you q a

THANK YOUQ&A


References

REFERENCES

  • Buza, K. and Thieme, L. S.: Motif-based Classification of Time Series with Bayesian Networks and SVMs. In: A. Fink et al. (eds.) Advances in Data Analysis, Data Handling and Business Intelligences, Studies in Classification, Data Analysis, Knowledge Organization. Springer-Verlag, pp. 105-114 (2010).

  • B. Chiu, E. Keogh, and S. Lonardi, Probabilistic Discovery of Time Series Motifs, Proceedings of the 9th International Conference on Knowledge Discovery and Data mining, Washington, D.C., 2003.

  • Gruber C., Coduro, M., Sick, B.: Signature Verification with Dynamic RBF Networks and Time Series Motifs. In : Proc of 10th Int. Workshop on Frontiers in Handwriting Recognition (2006).

  • J. Lin, E. Keogh, Patel, P. and Lonardi, S., Finding Motifs in Time Series, The 2nd Workshop on Temporal Data Mining, at the 8th ACM SIGKDD International Conference on Knowledge Discovery and Data Mining, Edmonton, Alberta, Canada, 2002.

  • Mueen, A., Keogh, E., Zhu, Q., Cash, S., and Westover, B.: Exact Discovery of Time Series Motifs. In: Proc. of SDM (2009).


Discovering motifs in time series

6. Jiang, Y., Li, C., Han, J.: Stock temporal prediction based on time series motifs. In: Proc. of 8th Int. Conf. on Machine Learning and Cybernetics, Baoding, China, July 12-15 (2009).

7. Li, Q., Lopez, I.F.V. and Moon, B.: Skyline Index for time series data, IEEE Trans. on Knowledge and Data Engineering, Vol. 16, No. 4 (2004)

8. Phu, L. and Anh, D. T., Motif-based Method for Initialization k-Means Clustering of Time Series Data, Proc. of 24th Australasian Joint Conference (AI 2011), Perth, Australia, Dec. 5-8. Dianhui Wang, Mark Reynolds (Eds.), LNAI 7106, Springer-Verlag, 2011, pp. 11-20.

9. Son, N.T., Anh, D.T., Discovering Approximate Time Series Motif based on MP_C Method with the Support of Skyline Index, Proc. of 4th Int. Conf. on Knowledge and Systems Engineering (KSE’2012), Aug. 17-19, Da Nang, Vietnam (to appear).

10. Nguyen Thanh Son, Duong Tuan Anh, Time Series Similarity Search based on Middle Points and Clipping,Proc. of 3rd Conference on Data Mining and Optimization (DMO), 27-29 June, 2011, Putrajaya, Malaysia, pp.13-19.

11. Xi, X., Keogh, E., Wei, L., Mafra-Neto, A., Finding Motifs in a Database of Shapes, Proc. of SIAM 2007, pp. 249-270.


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