1 Introduction
Over the past two decades, the graph Laplacian matrix and its variants have been widely adopted for solving various research tasks, including graph partitioning [23], data clustering [14], community detection [28, 5], consensus in networks [19], dimensionality reduction [2], entity disambiguation [33], link prediction [32], graph signal processing [27], centrality measures for graph connectivity [4], interconnected physical systems [24], network vulnerability assessment [7], image segmentation [26], among others. The fundamental task is to represent the data of interest as a graph for analysis, where a node represents an entity (e.g., a pixel in an image or a user in an online social network) and an edge represents similarity between two multivariate data samples or actual relation (e.g., friendship) between nodes [14]. More often the eigenvectors associated with the smallest eigenvalues of the graph Laplacian matrix are used to cluster the entities into clusters of high similarity. For brevity, throughout this paper we will call these eigenvectors as the smallest eigenvectors.
The success of graph Laplacian matrix based methods for graph partitioning and spectral clustering can be explained by the fact that acquiring smallest eigenvectors is equivalent to solving a relaxed graph cut minimization problem, which partitions a graph into clusters by minimizing various objective functions including min cut, ratio cut or normalized cut [14]. Generally, in clustering is selected to be much smaller than
(the number of data points), making full eigen decomposition (such as QR decomposition) unnecessary. An efficient alternative is to use methods that are based on power iteration, such as Arnoldi method or Lanczos method, which computes the leading eigenpairs through repeated matrix vector multiplication. ARPACK
[13] library is a popular parallel implementation of different variants of Arnoldi and Lanczos method, which is used by many commercial software including Matlab.However, in most situations the best value of is unknown and a heuristic is used by clustering algorithms to determine the number of clusters, e.g., fixing a maximum number of clusters and detecting a large gap in the values of the largest sorted eigenvalues or normalized cut score [21, 16]. Alternatively, this value of can be determined based on domain knowledge [1]. For example, a user may require that the largest cluster size be no more than 10% of the total number of nodes or that the total intercluster edge weight be no greater than a certain amount. In these cases, the desired choice of cannot be determined a priori
. Overestimation of the upper bound
on the number of clusters is expensive as the cost of finding eigenpairs using the power iteration method grows rapidly with . On the other hand, choosing an insufficiently large value for runs the risk of severe bias. Setting to the number of data points is generally computationally infeasible, even for a moderatesized graph. Therefore, an incremental eigenpair computation method that effectively computes the th smallest eigenpair of graph Laplacian matrix by utilizing the previously computed smallest eigenpairs is needed. Such an iterative method obviates the need to set an upper bound on , and its efficiency can be explained by the adaptivity to increments in .By exploiting the special matrix properties and graph characteristics of a graph Laplacian matrix, we propose an efficient method for computing the th eigenpair given all of the smallest eigenpairs, which we call the Incremental method of Increasing Orders (IncrementalIO). For each increment, given the previously computed smallest eigenpairs, we show that computing the next smallest eigenpair is equivalent to computing a leading eigenpair of a particular matrix, which transforms potentially tedious numerical computation (such as the iterative tridiagonalization and eigendecomposition steps in the Lanczos algorithm [11]) to simple matrix power iterations of known computational efficiency [10]. We then compare the performance of IncrementalIO with a batch computation method which computes all of the smallest eigenpairs in a single batch, and an incremental method adapted from the Lanczos algorithm, which we call the Lanczos method of Increasing Orders (LanczosIO). For a given number of eigenpairs iterative matrixvector multiplication of Lanczos procedure yields a set of Lanczos vectors (), and a tridiagonal matrix (), followed by a full eigendecomposition of ( is a value much smaller than the matrix size). LanczosIO saves the Lanczos vectors that were obtained while eigenpairs were computed and use those to generate new Lanczos vectors for computing th eigenpair.
Comparing to the batch method, our experimental results show that for a given order , IncrementalIO provides a significant reduction in computation time. Also, as increases, the gap between IncrementalIO and the batch approach widens, providing an order of magnitude speedup. Experiments on reallife datasets show that the performance of LanczosIO is overly sensitive to the selection of augmented Lanczos vectors, a parameter that cannot be optimized a priori—for some of our experimental datasets, LanczosIO performs even worse than the batch method (see Sec. 6). Moreover, LanczosIO consumes significant amount of memory as it has to save the Lanczos vectors () for making the incremental approach realizable. In summary, LanczosIO, although an incremental eigenpair computation algorithm, falls short in terms of robustness.
To illustrate the reallife utility of incremental eigenpair computation methods, we design a userguided spectral clustering algorithm which uses IncrementalIO. The algorithm provides clustering solution for a sequence of values efficiently, and thus enable a user to compare these clustering solutions for facilitating the selection of the most appropriate clustering.
The contributions of this paper are summarized as follows.

We propose an incremental eigenpair computation method (IncrementalIO) for both unnormalized and normalized graph Laplacian matrices, by transforming the original eigenvalue decomposition problem into an efficient sequential leading eigenpair computation problem. Simulation results show that IncrementalIO generates the desired eigenpair accurately and has superior performance over the batch computation method in terms of computation time.

We show that IncrementalIO is robust in comparison to LanczosIO, which is an incremental eigenpair method that we design by adapting the Lanczos method.

We use several reallife datasets to demonstrate the utility of IncrementalIO. Specifically, we show that IncrementalIO is suitable for userguided spectral clustering which provides a sequence of clustering results for unit increment of the number
of clusters, and updates the associated cluster evaluation metrics for helping a user in decision making.
2 Related Works
2.1 Incremental eigenvalue decomposition
The proposed method (IncrementalIO) aims to incrementally compute the smallest eigenpair of a given graph Laplacian matrix. There are several works that are named as incremental eigenvalue decomposition methods [17, 9, 18, 8, 25]. However, these works focus on updating the eigenstructure of graph Laplacian matrix of dynamic graphs when nodes (data samples) or edges are inserted or deleted from the graph, which are different from incremental computation of eigenpairs of increasing orders.
2.2 Cluster Count Selection for Spectral Clustering
Many spectral clustering algorithms utilize the eigenstructure of graph Laplacian matrix for selecting number of clusters. In [21], a value that maximizes the gap between the th and the th smallest eigenvalue is selected as the number of clusters. In [16], a value
that minimizes the sum of clusterwise Euclidean distance between each data point and the centroid obtained from Kmeans clustering on
smallest eigenvectors is selected as the number of clusters. In [31], the smallest eigenvectors of normalized graph Laplacian matrix are rotated to find the best alignment that reflects the true clusters. A model based method for determining the number of clusters is proposed in [22]. Note that aforementioned methods use only one single clustering metric to determine the number of clusters and often implicitly assume an upper bound on (namely ).3 Incremental Eigenpair Computation for Graph Laplacian Matrices
3.1 Background
Throughout this paper bold uppercase letters (e.g., ) denote matrices and (or ) denotes the entry in th row and th column of , bold lowercase letters (e.g., or ) denote column vectors, denotes matrix or vector transpose, italic letters (e.g., or ) denote scalars, and calligraphic uppercase letters (e.g., or ) denote sets. The vector of ones (zeros) is denoted by (). The matrix
denotes an identity matrix and the matrix
denotes the matrix of zeros.We use two symmetric matrices, and , to denote the adjacency and weight matrices of an undirected weighted simple graph with nodes and edges. if there is an edge between nodes and , and otherwise. is a nonnegative symmetric matrix such that if and if . Let denote the strength of node . Note that when , the strength of a node is equivalent to its degree. is a diagonal matrix with nodal strength on its main diagonal and the offdiagonal entries being zero.
The (unnormalized) graph Laplacian matrix is defined as
(1) 
One popular variant of the graph Laplacian matrix is the normalized graph Laplacian matrix defined as
(2) 
where . The th smallest eigenvalue and its associated unitnorm eigenvector of are denoted by and , respectively. That is, the eigenpair of has the relation , and . The eigenvectors have unit Euclidean norm and they are orthogonal to each other such that if and if . The eigenvalues of are said to be distinct if . Similar notations are used for .
3.2 Theoretical foundations of the proposed method (IncrementalIO)
The following lemmas and corollaries provide the cornerstone for establishing the proposed incremental eigenpair computation method (IncrementalIO). The main idea is that we utilize the eigenspace structure of graph Laplacian matrix to inflate specific eigenpairs via a particular perturbation matrix, without affecting other eigenpairs. IncrementalIO can be viewed as a specialized Hotelling’s deflation method
[20] designed for graph Laplacian matrices by exploiting their spectral properties and associated graph characteristics. It works for both connected, and disconnected graphs using both normalized and unnormalized graph Laplacian matrices. For illustration purposes, in Table 1 we group the established lemmas, corollaries, and theorems under different graph types and different graph Laplacian matrices. Because of the page limit, the proofs of the established lemmas, theorems and corollaries are given in the extended version^{1}^{1}1http://arxiv.org/abs/1512.07349.Lemma 1
Assume that is a connected graph and is the graph Laplacian with denoting the sum of the entries in the th row of the weight matrix . Let and define Then the eigenpairs of satisfy for and .
Corollary 1
For a normalized graph Laplacian matrix , assume is a connected graph and let . Then for and .
Lemma 2
Assume that is a disconnected graph with connected components, and let , let , and let
.
Then for ,
for , and
.
Corollary 2
For a normalized graph Laplacian matrix , assume is a disconnected graph with connected components. Let
, and let
Then
for ,
for , and
.
3.3 Incremental method of increasing orders
Given the smallest eigenpairs of a graph Laplacian matrix, we prove that computing the th smallest eigenpair is equivalent to computing the leading eigenpair (the eigenpair with the largest eigenvalue in absolute value) of a certain perturbed matrix. The advantage of this transformation is that the leading eigenpair can be efficiently computed via matrix power iteration methods [11, 13].
Let be the matrix with columns being the smallest eigenvectors of and let be the diagonal matrix with being its main diagonal. The following theorems show that given the smallest eigenpairs of , the th smallest eigenpair of is the leading eigenvector of the original graph Laplacian matrix perturbed by a certain matrix.
Theorem 1
(connected graphs)
Given and , assume that is a connected graph. Then the eigenpair
is a leading eigenpair of the matrix . In particular, if has distinct eigenvalues, then .
The next theorem describes an incremental eigenpair computation method when the graph is a disconnected graph with connected components. The columns of the matrix are the smallest eigenvectors of . Note that has a canonical representation that the nonzero entries of each column are a constant and their indices indicate the nodes in each connected component [14, 6], and the columns of are the smallest eigenvectors of with eigenvalue [6]. Since the smallest eigenpairs with the canonical representation are trivial by identifying the connected components in the graph, we only focus on computing the th smallest eigenpair given smallest eigenpairs, where . The columns of the matrix are the th to the th smallest eigenvectors of and the matrix is the diagonal matrix with being its main diagonal. If , and are defined as the matrix with all entries being zero, i.e., .
Theorem 2
(disconnected graphs) Assume that is a disconnected graph with connected components, given , and , the eigenpair is a leading eigenpair of the matrix . In particular, if has distinct nonzero eigenvalues, then .
Following the same methodology for proving Theorem 1 and using Corollary 1, for normalized graph Laplacian matrices, let be the matrix with columns being the smallest eigenvectors of and let . The following corollary provides the basis for incremental eigenpair computation for normalized graph Laplacian matrix of connected graphs.
Corollary 3
For the normalized graph Laplacian matrix of a connected graph , given and , the eigenpair is a leading eigenpair of the matrix . In particular, if has distinct eigenvalues, then
.
For disconnected graphs with connected components, let with columns being the th to the th smallest eigenvectors of and let . Based on Corollary 2, the following corollary provides an incremental eigenpair computation method for normalized graph Laplacian matrix of disconnected graphs.
Corollary 4
For the normalized graph Laplacian matrix of a disconnected graph with connected components, given , and ,
the eigenpair is a leading eigenpair of the matrix . In particular, if has distinct eigenvalues, then
.
3.4 Computational complexity analysis
Here we analyze the computational complexity of IncrementalIO and compare it with the batch computation method. IncrementalIO utilizes the existing smallest eigenpairs to compute the th smallest eigenpair as described in Sec. 3.3, whereas the batch computation method recomputes all eigenpairs for each value of . Both methods can be easily implemented via welldeveloped numerical computation packages such as ARPACK [13].
Following the analysis in [10], the average relative error of the leading eigenvalue from the Lanczos algorithm [11] has an upper bound of the order , where is the number of nodes in the graph and is the number of iterations for Lanczos algorithm. Therefore, when one sequentially computes from to smallest eigenpairs, for IncrementalIO the upper bound on the average relative error of smallest eigenpairs is since in each increment computing the corresponding eigenpair can be transformed to a leading eigenpair computation process as described in Sec. 3.3. On the other hand, for the batch computation method, the upper bound on the average relative error of smallest eigenpairs is since for the th increment () it needs to compute all smallest eigenpairs from scratch. These results also imply that to reach the same average relative error for sequential computation of smallest eigenpairs, IncrementalIO requires iterations, whereas the batch method requires iterations. It is difficult to analyze the computational complexity of LanczosIO, as its convergence results heavily depend on the quality of previously generated Lanczos vectors.
4 Application: UserGuided Spectral Clustering with IncrementalIO
Based on the developed incremental eigenpair computation method (IncrementalIO) in Sec. 3, we propose an incremental algorithm for userguided spectral clustering as summarized in Algorithm 1. This algorithm sequentially computes the smallest eigenpairs via IncrementalIO (steps 13) for spectral clustering and provides a sequence of clusters with the values of userspecified clustering metrics.
The input graph is a connected undirected weighted graph and we convert it to the reduced weighted graph to alleviate the effect of imbalanced edge weights. The entries of are properly normalized by the nodal strength such that . We then obtain the graph Laplacian matrix for and incrementally compute the eigenpairs of via IncrementalIO (steps 13) until the user decides to stop further computation.
Starting from clusters, the algorithm incrementally computes the th smallest eigenpair of with the knowledge of the previous smallest eigenpairs via Theorem 1 and obtains matrix containing smallest eigenvectors. By viewing each row in as a dimensional vector, Kmeans clustering is implemented to separate the rows in into clusters. For each increment, the identified clusters are denoted by , where is a graph partition with nodes and edges.
In addition to incremental computation of smallest eigenpairs, for each increment the algorithm can also be used to update clustering metrics such as normalized cut, modularity, and cluster size distribution, in order to provide users with clustering information to stop the incremental computation process. The incremental computation algorithm allows users to efficiently track the changes in clusters as the number of hypothesized clusters increases.
Note that Algorithm 1 is proposed for connected graphs and their corresponding unnormalized graph Laplacian matrices. The algorithm can be easily adapted to disconnected graphs or normalized graph Laplacian matrices by modifying steps 13 based on the developed results in Theorem 2, Corollary 3 and Corollary 4.
5 Implementation
We implement the proposed incremental eigenpair computation method (IncrementalIO) using Matlab R2015a’s “eigs” function, which is based on ARPACK package [13]. Note that this function takes a parameter and returns leading eigenpairs of the given matrix. The function is implemented in Matlab with a Lanczos algorithm that computes the leading eigenpairs (the implicitlyrestarted Lanczos method [3]). This Matlab function iteratively generates Lanczos vectors starting from an initial vector (the default setting is a random vector) with restart. Following Theorem 1, IncrementalIO works by sequentially perturbing the graph Laplacian matrix with a particular matrix and computing the leading eigenpair of the perturbed matrix (see Algorithm 1) by calling . For the batch computation method, we use to compute the desired eigenpairs from scratch as increases.
For implementing LanczosIO, we extend the Lanczos algorithm of fixed order ( is fixed) using the PROPACK package [12]. As we have stated earlier, LanczosIO works by storing all previously generated Lanczos vectors and using them to compute new Lanczos vectors for each increment in . The general procedure of computing leading eigenpairs of a real symmetric matrix using LanczosIO is described in Algorithm 2. The operation of LanczosIO is similar to the explicitlyrestarted Lanczos algorithm [29], which restarts the computation of Lanczos vectors with a subset of previously computed Lanczos vectors. Note that the LanczosIO consumes additional memory for storing all previously computed Lanczos vectors when compared with the proposed incremental method in Algorithm 1, since the function uses the implicitlyrestarted Lanczos method that spares the need of storing Lanczos vectors for restart.
To apply LanczosIO to spectral clustering of increasing orders, we can set to obtain the smallest eigenvectors of . Throughout the experiments the parameters in Algorithm 2 are set to be and , where is the machine precision, is the operator norm of , and these settings are consistent with the settings used in function [13]. The number of augmented Lanczos vectors is set to be , and the effect of on the computation time is discussed in Sec. 6.2. The Matlab implementation of the aforementioned batch method, LanczosIO, and IncrementalIO are available at https://sites.google.com/site/pinyuchenpage/codes.
6 Experimental Results
We perform several experiments: first, compare the computation time between IncrementalIO, LanczosIO, and the batch method; second, numerically verify the accuracy of IncrementalIO; third, demonstrate the usages of IncrementalIO for userguided spectral clustering. For the first experiment, we generate synthetic ErdosRenyi random graphs of various sizes. For the second experiment, we compare the consistency of eigenpairs obtained from IncrementalIO and the batch method. For the third experiment, we use six popular graph datasets as summarized in Table 2.
6.1 Comparison of computation time on simulated graphs
To illustrate the advantage of IncrementalIO, we compare its computation time with the other two methods, the batch method and LanczosIO, for varying order and varying graph size . The ErdosRenyi random graphs that we build are used for this comparison. Fig. 1 (a) shows the computation time of IncrementalIO, LanczosIO, and the batch computation method for sequentially computing from to smallest eigenpairs. It is observed that the computation time of IncrementalIO and LanczosIO grows linearly as increases, whereas the computation time of the batch method grows superlinearly with .
Fig. 1 (b) shows the computation time of all three methods with respect to different graph size . It is observed that the difference in computation time between the batch method and the two incremental methods grow exponentially as increases, which suggests that in this experiment IncrementalIO and LanczosIO are more efficient than the batch computation method, especially for large graphs. It is worth noting that although LanczosIO has similar performance in computation time as IncrementalIO, it requires additional memory allocation for storing all previously computed Lanczos vectors.
Dataset  Nodes  Edges  Density  




0.095%  



0.054%  
CLUTO^{4}^{4}4http://glaros.dtc.umn.edu/gkhome/views/cluto 


2.54%  



0.041%  
Youtube^{6}^{6}6http://socialcomputing.asu.edu/datasets/YouTube 


0.082%  
BlogCatalog^{7}^{7}7http://socialcomputing.asu.edu/datasets/BlogCatalog 


0.63% 
Sequential eigenpair computation time on ErdosRenyi random graphs with edge connection probability
. The marker represents averaged computation time of 50 trials and the error bar represents standard deviation. (a) Computation time with
and different number of eigenpairs . (b) Computation time with and different number of nodes .6.2 Comparison of computation time on reallife datasets
Fig. 3 shows the time improvement of IncrementalIO relative to the batch method for the reallife datasets listed in Table 2
, where the difference in computation time is displayed in log scale to accommodate large variance of time improvement across datasets that are of widely varying size. It is observed that the gain in computational time via IncrementalIO is more pronounced as cluster count
increases, which demonstrates the merit of the proposed incremental method.On the other hand, although LanczosIO is also an incremental method, in addition to the wellknown issue of requiring memory allocation for storing all Lanczos vectors, the experimental results show that it does not provide performance robustness as IncrementalIO does, as it can perform even worse than the batch method for some cases. Fig. 3 shows that LanczosIO actually results in excessive computation time compared with the batch method for four out of the six datasets, whereas in Fig. 3 IncrementalIO is superior than the batch method for all these datasets, which demonstrates the robustness of IncrementalIO over LanczosIO. The reason of lacking robustness for LanczosIO can be explained by the fact the previously computed Lanczos vectors may not be effective in minimizing the Ritz approximation error of the desired eigenpairs. In contrast, IncrementalIO and the batch method adopt the implicitlyrestarted Lanczos method, which restarts the Lanczos algorithm when the generated Lanczos vectors fail to meet the Ritz approximation criterion, and may eventually lead to faster convergence. Furthermore, Fig. 4 shows that LanczosIO is overly sensitive to the number of augmented Lanczos vectors , which is a parameter that cannot be optimized a priori.
Theorem 1 establishes that the proposed incremental method exactly computes the th eigenpair using 1 to th eigenpairs, yet for the sake of experiments with real datasets, we have computed the normed eigenvalue difference and eigenvector correlation of the smallest eigenpairs using the batch method and IncrementalIO as displayed in Fig. 6. The smallest eigenpairs are identical as expected; to be specific, using Matlab library, on the Minnesota road dataset for , the normed eigenvalue difference is and the associated eigenvectors are identical up to differences in sign. Results for the other datasets are reported in the extended version^{1}^{1}footnotemark: 1.
6.3 Clustering metrics for userguided spectral clustering
In reallife, an analyst can use IncrementalIO for clustering along with a mechanism for selecting the best choice of starting from . To demonstrate this, in the experiment we use five clustering metrics that can be used for online decision making regarding the value of . These metrics are commonly used in clustering unweighted and weighted graphs and they are summarized as follows.
1. Modularity: modularity is defined as
(3) 
where is the set of all nodes in the graph, is the th cluster, () denotes the sum of weights of all internal (external) edges of the th cluster, , and denotes the total nodal strength.
2. Scaled normalized cut (SNC): NC is defined as [30]
(4) 
SNC is NC divided by the number of clusters, i.e., NC/.
3. Scaled median (or maximum) cluster size: Scaled medium (maximum) cluster size is the medium (maximum) cluster size of clusters divided by the total number of nodes of a graph.
4. Scaled spectrum energy: scaled spectrum energy is the sum of the smallest eigenvalues of the graph Laplacian matrix divided by the sum of all eigenvalues of , which can be computed by
(5) 
where is the th smallest eigenvalue of and
is the sum of diagonal elements of .
These metrics provide alternatives for gauging the quality of the clustering method. For example, Mod and NC reflect the tradeoff between intracluster similarity and intercluster separation. Therefore, the larger the value of Mod, the better the clustering quality, and the smaller the value of NC, the better the clustering quality. Scaled spectrum energy is a typical measure of cluster quality for spectral clustering [21, 16, 31], and smaller spectrum energy means better separability of clusters. For Mod and scaled NC, a user might look for a cluster count such that the increment in the clustering metric is not significant, i.e., the clustering metric is saturated beyond such a . For scaled median and maximum cluster size, a user might require the cluster count to be such that the clustering metric is below a desired value. For scaled spectrum energy, a user might look for a noticeable increase in the clustering metric between consecutive values of .
6.4 Demonstration
Here we use Minnesota Road data to demonstrate how users can utilize the clustering metrics in Sec. 6.3 to determine the number of clusters. The five metrics evaluated for Minnesota Road clustering with respect to different cluster counts are displayed in Fig. 6. Starting from clusters, these metrics are updated by the incremental userguided spectral clustering algorithm (Algorithm 1) as increases. If the user imposes that the maximum cluster size should be less than of the total number of nodes, then the algorithm returns clustering results with a number of clusters of or greater. Inspecting the modularity one sees it saturates at , and the user also observes a noticeable increase in scaled spectrum energy when . Therefore, the algorithm can be used to incrementally generate four clustering results for , and . The selected clustering results in Fig. 7 are shown to be consistent with geographic separations of different granularity.
We also apply the proposed incremental userguided spectral clustering algorithm (Algorithm 1) to Power Grid, CLUTO, Swiss Roll, Youtube, and BlogCatalog. In Fig.8, we show how the value of clustering metrics changes with , for each dataset. The incremental method enables us to efficiently generate all clustering results with and so on. Due to space limitation, for each dataset we only show the trend of the three clustering metrics that exhibit the highest variation for different ; thus, the chosen clustering metrics can be different for different datasets. This suggests that selecting the correct number of clusters is a difficult task and a user might need to use different clustering metrics for a range of values, and IncrementalIO is an effective tool to support such an endeavor.
7 Conclusion
In this paper we present IncrementalIO, an efficient incremental eigenpair computation method for graph Laplacian matrices which works by transforming a batch eigenvalue decomposition problem into a sequential leading eigenpair computation problem. The method is elegant, robust and easy to implement using a scientific programming language, such as Matlab. We provide analytical proof of its correctness. We also demonstrate that it achieves significant reduction in computation time when compared with the batch computation method. Particularly, it is observed that the difference in computation time of these two methods grows exponentially as the graph size increases.
To demonstrate the effectiveness of IncrementalIO, we also show experimental evidences that obtaining such an incremental method by adapting the existing leading eigenpair solvers (such as, the Lanczos algorithm) is nonobvious and such efforts generally do not lead to a robust solution.
Finally, we demonstrate that the proposed incremental eigenpair computation method (IncrementalIO) is an effective tool for a userguided spectral clustering task, which effectively updates clustering results and metrics for each increment of the cluster count.
Acknowledgments
This work is partially supported by the Consortium for Verification Technology under Department of Energy National Nuclear Security Administration award number DENA0002534, by ARO grant W911NF1510479, by ARO grant W911NF1510241, and by Mohammad Hasan’s NSF CAREER Award (IIS1149851).
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8 Supplementary File
8.1 Proof of Lemma 1
Since is a positive semidefinite (PSD) matrix, for all . Since is a connected graph, by (1) . Therefore, by the PSD property we have . Moreover, since is a symmetric realvalued square matrix, from (1) we have
(6) 
By the PSD property of , we have since for any connected graph. Therefore, by the orthogonality of eigenvectors of (i.e., for all ) the eigenvalue decomposition of can be represented as
(7) 
where and
for .
8.2 Proof of Lemma 2
The graph Laplacian matrix of a disconnected graph consisting of connected components can be represented as a matrix with diagonal block structure, where each block in the diagonal corresponds to one connected component in [6], i.e.,
(8) 
where is the graph Laplacian matrix of th connected component in . From the proof of Lemma 1 each connected component contributes to exactly one zero eigenvalue for , and
(9) 
Therefore, we have the results in Lemma 2.
8.3 Proof of Corollary 1
8.4 Proof of Corollary 2
8.5 Proof of Theorem 1
From Lemma 1,
(10) 
which is a valid eigenpair decomposition that can be seen by inflating the smallest eigenvalues of to with the originally paired eigenvectors. Using (8.5) we obtain the eigenvalue decomposition of as
(11) 
Since , we have . Therefore, the eigenpair can be obtained by computing the leading eigenpair of . In particular, if has distinct eigenvalues, then the leading eigenpair of is unique. Therefore, by (8.5) we have the relation
(12) 
8.6 Proof of Theorem 2
First observe from (8) that has zero eigenvalues since each connected component contributes to exactly one zero eigenvalue for . Following the same derivation procedure in the proof of Theorem 1 and using Lemma 2, we have
(13) 
Therefore, the eigenpair can be obtained by computing the leading eigenpair of . If has distinct nonzero eigenvalues (i.e, ), we obtain the relation .
8.7 Complete clustering metrics for userguided spectral clustering
8.8 Demonstration of equivalence between the proposed incremental computation method and the batch computation method
Here we use the smallest eigenpairs of the datasets in Table 2 computed by the batch method and IncrementalIO to verify Theorem 1, which proves that IncrementalIO exactly computes the th eigenpair using 1 to th eigenpairs. Fig. 10 shows the normed eigenvalue differences (in terms of root mean squared error) and the correlations of the associated eigenvectors obtained from the two methods. It can be observed that the normed eigenvalue difference is negligible and the associated eigenvectors are identical up to the difference in sign, i.e., the eigenvector correlation in magnitude equals to 1 for every pair of corresponding eigenvectors of the two methods.
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