# Leader Selection in Stochastically Forced Consensus Networks

 June 2013 Matlab Files Download all Matlab files Presentation 2013 Allerton Talk

## Purpose

This website provides a Matlab implementation of algorithms that quantify suboptimality of solutions to two combinatorial network optimization problems. For undirected consensus networks, these problems arise in assignment of a pre-specified number of nodes, as leaders, in order to minimize the mean-square deviation from consensus. For distributed localization in sensor networks, one of our formulations is equivalent to the problem of choosing a pre-specified number of absolute position measurements among available sensors in order to minimize the variance of the estimation error.

## Two combinatorial problems involving graph Laplacian

### Diagonally strengthened graph Laplacian

Let be the graph Laplacian of an undirected connected network. A diagonally strengthened graph Laplacian is obtained by adding a vector with positive elements to the main diagonal of ,

For connected graphs, the diagonally strengthened Laplacian is a positive definite matrix.

We are interested in selecting a pre-specified number of diagonal elements of such that the trace of the inverse of the strengthened graph Laplacian is minimized. This problem can be expressed as

where the graph Laplacian and the vector with positive elements are problem data, and the Boolean-valued vector with cardinality is the optimization variable.

This combinatorial optimization problem is of interest in several applications. For example, in leader-follower networks subject to stochastic disturbances, the problem amounts to assigning nodes as leaders (that have access to their own states) such that the mean-square deviation from consensus is minimized. We refer to as the noise-corrupted leader selection problem. Also, it can be shown that the problem of choosing absolute position measurements among sensors in order to minimize the variance of the estimation error in sensor networks is equivalent to the problem .

### Principal submatrices of graph Laplacian

Let be a principal submatrix of the graph Laplacian obtained by deleting a pre-specified number of rows and columns. For undirected connected graphs, is a positive definite matrix.

The problem of finding a principal submatrix of such that is minimized can be expressed as

where the Boolean-valued vector with cardinality is the optimization variable. The index set of nonzero elements of identifies the rows and columns of the graph Laplacian that need to be deleted in order to obtain .

Similar to , the combinatorial optimization problem arises in several applications. For example, in leader-follower networks, the problem amounts to assigning nodes as leaders (that perfectly follow their desired trajectories) such that the mean-square deviation from consensus is minimized. We refer to as the noise-free leader selection problem.

## Performance bounds on the global optimal value

For large graphs, it is challenging to determine global solutions to the combinatorial optimization problems and . Instead, we develop efficient algorithms that quantify performance bounds on the global optimal values. Specifically, we obtain lower bounds by solving convex relaxations of and and we obtain upper bounds using a simple but efficient greedy algorithm. We note that the greedy approach also provides feasible points of and .

### Lower bounds from convex relaxations

Since the objective function in is the composition of a convex function of a positive definite matrix with an affine function , it follows that is a convex function of . By enlarging the Boolean constraint set to its convex hull , we obtain the following convex relaxation of :

Schur complement can be used to cast as a semidefinite program (SDP). Thus, can be solved using general-purpose SDP solvers with computational complexity of order . In contrast, computational complexity of a customized interior point method that we develop is of order .

In contrast to , the objective function in is a nonconvex function of . By a change of optimization variables, we identify the source of nonconvexity in the form of a rank constraint. Removal of the rank constraint and relaxation of the Boolean constraints can be used to obtain the following convex relaxation of :

Here, the symmetric matrix and the vector are the optimization variables, is the elementwise multiplication of two matrices, is the vector of all ones, and the integer is given by .

Similar to , the convex relaxation can be cast as an SDP and solved using general-purpose SDP solvers. However, since this approach requires the computational complexity of order , we develop an alternative approach that is well-suited for large problems. Our approach relies on the use of the alternating direction method of multipliers (ADMM) which allows us to decompose into a sequence of subproblems that can be solved with operations.

### Upper bounds using a greedy algorithm

With lower bounds on the global optimal value resulting from the convex relaxations and , we use a greedy algorithm to compute upper bounds. This algorithm selects one leader at a time by assigning the node that provides the largest performance improvement as the leader. Once this is done, an attempt to improve a selection of leaders is made by checking possible swaps between the leaders and the followers. In both steps, substantial improvement in algorithmic complexity is achieved by exploiting structure of the low-rank modifications to Laplacian matrices.

Detailed description of the aforementioned algorithms (i.e., the greedy algorithm, the customized interior point method for , and the ADMM-based algorithm for ) are provided in our paper

## Description of the main function

Matlab Syntax
```>> [Jlow,Jup,x] = leaders(L,Nl,kappa,flag);
```

Our Matlab function leaders.m takes the graph Laplacian , the positive integer , the vector with positive elements, and the indicator variable

•  –  for the noise-corrupted leader selection formulation ;

•  –  for the noise-free leader selection formulation .

leaders.m returns the lower and upper bounds on the global optimal value of the leader selection problem or , along with the Boolean-valued vector which represents a feasible point for the selection of leaders.

## Acknowledgements

This project is supported in part by the National Science Foundation under

• CAREER Award CMMI-0644793

• Award CMMI-0927509

• Award CMMI-0927720.