# O(log n) each column, stacking columns
    sparse_1 = sp.rand(n, 1, density=np.log(n)/n)
    for i in range(m-1):
        sparse_1 = sp.hstack([sparse_1, sp.rand(n, 1, density=np.log(n)/(2*n))])

    # O(log m) each row, stacking rows
    sparse_2 = sp.rand(1, m, density=np.log(m)/m)
    for j in range(n-1):
        sparse_2 = sp.vstack([sparse_2, sp.rand(1,m, density=np.log(m)/(2*m))])

    A  = csr_matrix(sparse_1 + sparse_2)