
Sparse covariance estimation for Gaussian variables
===================================================

A derivative work by Judson Wilson, 5/22/2014. Adapted (with significant
improvements and fixes) from the CVX example of the same name, by Joelle
Skaf, 4/24/2008.

Topic References:

-  Section 7.1.1, Boyd & Vandenberghe “Convex Optimization”

Introduction
------------

Suppose :math:`y \in \mathbf{\mbox{R}}^n` is a Gaussian random variable
with zero mean and covariance matrix
:math:`R = \mathbf{\mbox{E}}[yy^T]`, with sparse inverse
:math:`S = R^{-1}` (:math:`S_{ij} = 0` means that :math:`y_i` and
:math:`y_j` are conditionally independent). We want to estimate the
covariance matrix :math:`R` based on :math:`N` independent samples
:math:`y_1,\dots,y_N` drawn from the distribution, and using prior
knowledge that :math:`S` is sparse

A good heuristic for estimating :math:`R` is to solve the problem

.. math::

   \begin{array}{ll}
       \mbox{maximize}   & \log \det(S) - \mbox{tr}(SY) \\
       \mbox{subject to} & \sum_{i=1}^n \sum_{j=1}^n |S_{ij}| \le \alpha \\
                         & S \succeq 0,
       \end{array}

where :math:`Y` is the sample covariance of :math:`y_1,\dots,y_N`, and
:math:`\alpha` is a sparsity parameter to be chosen or tuned.

Generate problem data
---------------------

.. code:: python

    import cvxpy as cp
    import numpy as np
    import scipy as scipy
    
    # Fix random number generator so we can repeat the experiment.
    np.random.seed(0)
    
    # Dimension of matrix.
    n = 10
    
    # Number of samples, y_i
    N = 1000
    
    # Create sparse, symmetric PSD matrix S
    A = np.random.randn(n, n)  # Unit normal gaussian distribution.
    A[scipy.sparse.random_array((n, n), density=0.85).todense().nonzero()] = 0  # Sparsen A
    Strue = A.dot(A.T) + 0.05 * np.eye(n)  # Force strict pos. def.
    
    # Create the covariance matrix associated with S.
    R = np.linalg.inv(Strue)
    
    # Create samples y_i from the distribution with covariance R. 
    y_sample = scipy.linalg.sqrtm(R).dot(np.random.randn(n, N))
    
    # Calculate the sample covariance matrix.
    Y = np.cov(y_sample)

Solve for several :math:`\alpha` values
---------------------------------------

.. code:: python

    # The alpha values for each attempt at generating a sparse inverse cov. matrix.
    alphas = [10, 2, 1]
    
    # Empty list of result matrixes S
    Ss = []
    
    # Solve the optimization problem for each value of alpha.
    for alpha in alphas:
        # Create a variable that is constrained to the positive semidefinite cone.
        S = cp.Variable(shape=(n,n), PSD=True)
        
        # Form the logdet(S) - tr(SY) objective. Note the use of a set
        # comprehension to form a set of the diagonal elements of S*Y, and the
        # native sum function, which is compatible with cvxpy, to compute the trace.
        # TODO: If a cvxpy trace operator becomes available, use it!
        obj = cp.Maximize(cp.log_det(S) - sum([(S*Y)[i, i] for i in range(n)]))
        
        # Set constraint.
        constraints = [cp.sum(cp.abs(S)) <= alpha]
        
        # Form and solve optimization problem
        prob = cp.Problem(obj, constraints)
        prob.solve(solver=cp.CVXOPT)
        if prob.status != cp.OPTIMAL:
            raise Exception('CVXPY Error')
    
        # If the covariance matrix R is desired, here is how it to create it.
        R_hat = np.linalg.inv(S.value)
        
        # Threshold S element values to enforce exact zeros:
        S = S.value
        S[abs(S) <= 1e-4] = 0
    
        # Store this S in the list of results for later plotting.
        Ss += [S]
    
        print('Completed optimization parameterized by alpha = {}, obj value = {}'.format(alpha, obj.value))


.. parsed-literal::

    Completed optimization parameterized by alpha = 10, obj value = -16.167608186713004
    Completed optimization parameterized by alpha = 2, obj value = -22.545759632606043
    Completed optimization parameterized by alpha = 1, obj value = -26.989407069609157


Result plots
------------

.. code:: python

    import matplotlib.pyplot as plt
    
    # Show plot inline in ipython.
    %matplotlib inline
    
    # Plot properties.
    plt.rc('text', usetex=True)
    plt.rc('font', family='serif')
    
    # Create figure.
    plt.figure()
    plt.figure(figsize=(12, 12))
    
    # Plot sparsity pattern for the true covariance matrix.
    plt.subplot(2, 2, 1)
    plt.spy(Strue)
    plt.title('Inverse of true covariance matrix', fontsize=16)
    
    # Plot sparsity pattern for each result, corresponding to a specific alpha.
    for i in range(len(alphas)):
        plt.subplot(2, 2, 2+i)
        plt.spy(Ss[i])
        plt.title('Estimated inv. cov matrix, $\\alpha$={}'.format(alphas[i]), fontsize=16)



.. parsed-literal::

    <Figure size 432x288 with 0 Axes>



.. image:: sparse_covariance_est_files/sparse_covariance_est_5_1.png

