From ba26cc2ae34e6e442cccab9743d8389d9b009b6b Mon Sep 17 00:00:00 2001
From: Eric Teunis de Boone <ericteunis@gmail.com>
Date: Thu, 27 Feb 2020 18:35:42 +0100
Subject: [PATCH] Week4: Wrap up, almost finished

---
 week4/ex1.py | 183 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 183 insertions(+)
 create mode 100755 week4/ex1.py

diff --git a/week4/ex1.py b/week4/ex1.py
new file mode 100755
index 0000000..e0284a7
--- /dev/null
+++ b/week4/ex1.py
@@ -0,0 +1,183 @@
+#!/usr/bin/env python3
+
+import numpy as np
+from itertools import count
+
+
+def example_array():
+    return np.array([[ 3, 2, 1],
+                     [ 2, 3, 2],
+                     [ 1, 2, 3]], dtype=np.float64)
+
+def inverse_power_method(A, sigma, eps, max_iter=None):
+    """Determine an eigenvector of matrix A with an eigenvalue closest to sigma.
+
+    A       -- an N by N square matrix
+    sigma   -- parameter to select eigenvalue
+    eps     -- accuracy
+    """
+    A = np.asarray(A)
+    N = len(A)
+    B = np.linalg.inv( A - sigma*np.identity(N))
+    b_0 = np.ones(N)
+
+
+    for i in count():
+        b_1 = B @ b_0
+
+        b_1 = b_1 / np.linalg.norm(b_1)
+        
+        error = np.sqrt(np.sum( (np.abs(b_0) - np.abs(b_1))**2 ))
+
+        if error < eps:
+            return b_1, i
+
+        # Running out of iterations
+        if max_iter is not None and max_iter >= i:
+            raise RuntimeError("Not converging in {} steps".format(i))
+
+        # Setup for next loop
+        b_0 = b_1
+
+
+def test_inverse_power_method():
+    A = example_array() 
+    sigma = 1
+    eps = 1e-4
+    max_iter = 1e5
+    eigvec, n = inverse_power_method(A, sigma, eps, None)
+
+    _, npeigvecs = np.linalg.eig(A)
+   
+    rtol = 1e-2
+    found = False
+    for npeigvec in npeigvecs.T:
+        if np.allclose(npeigvec, eigvec, rtol):
+            found = True
+
+    assert found == True
+
+
+def tridiagonalize(A):
+    """ Compute the Tridiagonal of the matrix A. """
+    return householder_tridiagonalize(A)
+
+def householder_tridiagonalize(A):
+    """ Compute the Tridiagonal of the matrix A using the Householder Method. """
+
+    A = np.asarray(A)
+
+    N = len(A)
+
+    for k in range(N-1):
+        q = np.sqrt(np.sum( A[k+1:,k]**2 ))
+        alpha = -q * np.sign(A[k+1,k])
+        r = np.sqrt( (alpha**2 - A[k+1,k]*alpha)/2 )
+
+        v = np.zeros(N)
+        v[k+1] = (A[k+1, k] - alpha)
+        v[k+2:] = A[k+2:, k]
+        v[k + 1:] /= 2 * r
+
+
+        P = np.identity(N) - 2 * np.outer(v,v)
+
+        A = P @ A @ P
+
+    return A
+
+
+
+def test_tridiagonalize():
+    """ Compare the householder tridiagonalize function with Numpy's Hessenberg. """
+    from scipy.linalg import hessenberg
+
+    A = example_array() 
+    T = tridiagonalize(A)
+    H = hessenberg(A)
+
+    rtol = 1e-3
+    print(T)
+    print(H)
+    print(np.isclose(H, T, rtol))
+
+    print("They are almost the same except for a minus sign at (2,3) and (3,2)")
+
+
+def QREig(T, eps, max_iter=None):
+    """Compute Eigenvalues using QR decomposition."""
+    for i in count():
+        Q, R = np.linalg.qr(T)
+
+        T = R @ Q
+
+        # We are done if the sub-diagonal is smaller than eps
+        d_1 = np.diag(T, -1)
+        if np.linalg.norm(d_1) < eps:
+            return np.diagonal(T), i
+
+        # Running out of iterations
+        if max_iter is not None and max_iter >= i:
+            raise RuntimeError("Not converging in {} steps".format(i))
+
+def test_QREig():
+    A = example_array()
+
+    T = tridiagonalize(A)
+    eps = 1e-6
+
+    eigvals, _ = QREig(T, eps)
+    npeigvals, npeigvecs = np.linalg.eig(T)
+
+    assert np.allclose(eigvals, npeigvals, eps), "The computated eigenvalues do not match Numpy's eigenvalues"
+
+
+def eig(A, QReps, PMeps, sigma=1e-2):
+    """Determine eigenvectors with a combination of QR decomposition and inverse Power Method."""
+
+    T = tridiagonalize(A)
+    eigvals, _ = QREig(T, QReps)
+    eigvecs = np.zeros((len(eigvals),len(A)), dtype=np.float64)
+
+    sigma = (2*np.random.rand() -1 )*sigma
+    for i, eigval in enumerate(eigvals):
+        eigvecs[i], _ = inverse_power_method(T, eigval + sigma, PMeps)
+
+    return eigvals, eigvecs 
+
+
+def test_eig(debug=True):
+    A = example_array()
+
+    eps = 1e-5
+    rtol = 1e-1
+    eigvals, eigvecs = eig(A, eps, eps)
+
+    if debug:
+        for i, eigval in enumerate(eigvals):
+            print("Eigval {}: ".format(eigval), eigvecs[i])
+
+    npeigvals, npeigvecs = np.linalg.eig(A)
+    if debug:
+        for i, npeigval in enumerate(npeigvals):
+            print("Numpy Eigval {}: ".format(npeigval), npeigvecs[i])
+
+    assert np.allclose(eigvals, npeigvals, eps), "The computated eigenvalues do not match Numpy's eigenvalues"
+
+    #assert np.allclose(eigvecs, npeigvecs, rtol),  "The computated eigenvectors do not match Numpy's eigenvectors"
+
+    found = np.zeros(len(eigvecs))
+    for i, eigvec in enumerate(eigvecs):
+        for npeigvec in npeigvecs:
+            if np.allclose(eigvec, npeigvec, rtol):
+                found[i] = 1
+                break
+
+    print("Currently only the first eigenvector is the same from Numpy and my implementation. The others have a minus sign.")
+    print("Comparable eigenvectors: ", found)
+
+if __name__ == "__main__":
+    #test_inverse_power_method()
+    #test_tridiagonalize()
+    #test_QREig()
+    test_eig()