 # Newton’s Method applied to a scalar function Newton’s method for minimizing f(x): Twice differentiable function f(x), initial solution x 0. Generate a.

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Newton’s Method applied to a scalar function Newton’s method for minimizing f(x): Twice differentiable function f(x), initial solution x 0. Generate a sequence of solutions x 1, x 2, …and stop if the sequence converges to a solution with  f(x)=0. 1.Solve -  f(x k ) ≈  2 f(x k )  x 2. Let x k+1 =x k +  x. 3. let k=k+1

Newton’s Method applied to LS Not directly applicable to most nonlinear regression and inverse problems (not equal # of model parameters and data points, no exact solution to G(m)=d). Instead we will use N.M. to minimize a nonlinear LS problem, e.g. fit a vector of n parameters to a data vector d. f(m)=∑ [(G(m) i -d i )/  i ] 2 Let f i (m)=(G(m) i -d i )/  i i=1,2,…,m, F(m)=[f 1 (m) … f m (m)] T So that f(m)= ∑ f i (m) 2  f(m)=∑  f i (m) 2 ] m i=1 m i=1 m i=1

NM: Solve -  f(m k ) ≈  2 f(m k )  m LHS:  f(m k ) j = -∑ 2  f i (m k ) j F(m) j = -2 J(m k )F(m k ) RHS:  2 f(m k )  m = [2J(m) T J(m)+Q(m)]  m, where Q(m)= 2 ∑ f i (m)   f i (m) -2 J(m k )F(m k ) = 2 H(m)  m  m = -H -1 J(m k )F(m k ) = -H -1  f(m k ) (eq. 9.19) H(m)= 2J(m) T J(m)+Q(m) f i (m)=(G(m) i -d i )/  i i=1,2,…,m, F(m)=[f 1 (m) … f m (m)] T

Gauss-Newton (GN) method  2 f(m k )  m = H(m)  m = [2J(m k ) T J(m k )+Q(m)]  m ignores Q(m)=2∑ f i (m)   f i (m) :   f(m)≈2J(m) T J(m), assuming f i (m) will be reasonably small as we approach m*. That is, Solve -  f(x k ) ≈  2 f(x k )  x  f(m) j =∑ 2  f i (m) j F(m) j, i.e. J(m k ) T J(m k )  m=-J(m k ) T F(m k ) f i (m)=(G(m) i -d i )/  i i=1,2,…,m, F(m)=[f 1 (m) … f m (m)] T

Newton’s Method applied to LS Levenberg-Marquardt (LM) method uses [J(m k ) T J(m k )+ I]  m=-J(m k ) T F(m k ) ->0 : GN ->large, steepest descent (SD) (down-gradient most rapidly). SD provides slow but certain convergence. Which value of to use? Small values when GN is working well, switch to larger values in problem areas. Start with small value of, then adjust.

Steepest descent (SD) Problem of minimizing a quadratic function f(x) = 0.5 x T Ax − b T x, (equivalent to solving Ax=b) where b is in R n, A is an n x n symmetric positive definite matrix. The gradient of f(x) is Ax-b, i.e. x k+1 =x k -α k (Ax k -b) where α k is chosen to minimize f(x) along the direction of the negative gradient.

Alternative: line search

Example of slow convergence for SD Rosenbrock function: f(x 1,x 2 ) = (1 – x 1 ) 2 + 100 (x 2 – x 1 2 ) 2

Statistics of iterative methods Cov(Ad)=A Cov(d) A T (d has multivariate N.D.) Cov(m L2 )=(G T G) -1 G T Cov(d) G(G T G) -1 Cov(d)=  2 I: Cov(m L2 )=  2 (G T G) -1 However, we don’t have a linear relationship between data and estimated model parameters for the nonlinear regression, so cannot use these formulas. Instead: F(m*+  m)≈F(m*)+J(m*)  m Cov(m*)≈(J(m*) T J(m*)) -1 not exact due to linearization, confidence intervals may not be accurate r i =G(m*) i -d i,  i =1 s=[∑ r i 2 /(m-n)] Cov(m*)=s 2 (J(m*) T J(m*)) -1 establish confidence intervals,  2

Implementation Issues 1.Explicit (analytical) expressions for derivatives 2.Finite difference approximation for derivatives 3.When to stop iterating?  f(m)~0 ||m k+1 -m k ||~0, |f(m) k+1 -f(m) k |~0 eqs 9.47-9.49 4. Multistart method to optimize globally

Iterative Methods SVD impractical when matrix has 1000’s of rows and columns e.g., 256 x 256 cell tomography model, 100,000 ray paths, < 1% ray hits 50 Gb storage of system matrix, U ~ 80 Gb, V ~ 35 Gb Waste of storage when system matrix is sparse Iterative methods do not store the system matrix Provide approximate solution, rather than exact using Gaussian elimination Definition of iterative method: Starting point x 0, do steps x 1, x 2, … Hopefully converge toward right solution x

Iterative Methods Kaczmarz’s algorithm: Each of m rows of G i.m = d i define an n-dimensional hyperplane in R m 1)Project initial m(0) solution onto hyperplane defined by first row of G 2)Project m(1) solution onto hyperplane defined by second row of G 3)… until projections have been done onto all hyperplanes 4)Start new cycle of projections until converge If Gm=d has a unique solution, Kaczmarz’s algorithm will converge towards this If several solutions, it will converge toward the solution closest to m(0) If m(0)=0, we obtain the minimum length solution If no exact solution, converge fails, bouncing around near approximate solution If hyperplanes are nearly orthogonal, convergence is fast If hyperplanes are nearly parallel, convergence is slow

Let G i. be the ith row of G Consider the hyperplane defined by G i+1. m = d i+1. Since G T i+1. is perpendicular to this hyperplane, the update to m(i) from the constraint due to row i+1 of G will be proportional to G T i+1. m (i+1) = m (i) +βG T i+1. Since G i+1. m (i+1) = d i+1 then G i+1. (m (i) +βG T i+1. ) = d i+1 G i+1. m (i) - d i+1 = - βG i+1. G T i+1. β = [G i+1. m (i) - d i+1 ] / [G i+1. G T i+1. ] Kaczmarz’s algorithm:

1)Let m (0) = 0 2)For i=0,1,…,m, let m (i+1) = m (i) - G T i+1. [G i+1. m (i) - d i+1 ] / [G i+1. G T i+1. ] 3) If the solution has not yet converged, go back to step 1 Kaczmarz’s algorithm:

Kaczmarz’s Method y=x-1 y=1 y x

Specialized for tomography Kaczmarz: m (i+1) = m (i) - G T i+1. [G i+1. m (i) - d i+1 ] / [G i+1. G T i+1. ] Thus always adding a multiple of a row of G to the current solution G i+1. m (i) - d i+1 ] / [G i+1. G T i+1. is a normalized error in equation for i+1, and the correction is spread over the elements of m appearing in equation i+1 Often used approximation in ART is to replace all non-zero elements in row i+1 of G with ones, thus smearing the needed correction in traveltime equally over all cells in ray path i+1 SIRT Algorithm: ART with higher accuracy, each cell updated separately ART algorithm:

function x=kac(A,b,tolx,maxiter) [m,n]=size(A); AP=A’; x=zeros(n,1); iter=0; n2=zeros(m,1); for i=1:m n2(i)=norm(AP(:,i),2)^2; end while (iter <= maxiter) iter=iter+1; newx=x; for i=1:m newx=newx-((newx'*AP(:,i)-b(i))/(n2(i)))*AP(:,i); end if (norm(newx-x)/(1+norm(x)) < tolx) x=newx; return; end x=newx; end disp('Max iterations exceeded.'); return; Kaczmarz’s Algorithm

function x=sirt(A,b,tolx,maxiter) alpha=1.0; [m,n]=size(A); alpha=1.0; A1=(A>0); AP=A’; A1P=A1'; x=zeros(n,1); iter=0; N=zeros(m,1); L=zeros(m,1); NRAYS=zeros(n,1); for i=1:m N(i)=sum(A1P(:,i)); L(i)=sum(AP(:,i)); end for i=1:n NRAYS(i)=sum(A1(:,i)); end while (1==1) iter=iter+1; if (iter > maxiter) disp('Max iterations exceeded.'); return; end newx=x; deltax=zeros(n,1); for i=1:m, q=A1P(:,i)'*newx; delta=b(i)/L(i)-q/N(i); deltax=deltax+delta*A1P(:,i); end newx=newx+alpha*deltax./NRAYS; if (norm(newx-x)/(1+norm(x)) < tolx) return; end; x=newx; end SIRT Algorithm

function x=art(A,b,tolx,maxiter) alpha=1.0; [m,n]=size(A); A1=(A>0); AP=A’; A1P=A1’; x=zeros(n,1); iter=0; N=zeros(m,1); L=zeros(m,1); for i=1:m N(i)=sum(A1(i,:)); L(i)=sum(A(i,:)); end while (1==1) iter=iter+1; if (iter > maxiter) disp('Max iterations exceeded.'); x=newx; return; end newx=x; for i=1:m q=A1P(:,i)'*newx; delta=b(i)/L(i)-q/N(i); newx=newx+alpha*delta*A1P(:,i); end if (norm(newx-x)/(1+norm(x)) < tolx) x=newx; return; end x=newx; end ART Algorithm

True Model Reconstruction From Kaczmarz, ART, SIRT (all similar)

Conjugate Gradients Method Symmetric, positive definite system of equations Ax=b min  (x) = min(1/2 x T Ax - b T x)  (x) = Ax - b = 0 or Ax = b CG method: construct basis p 0, p 1, …, p n-1 such that p i T Ap j =0 when i≠j. Such basis is mutually conjugate w/r to A. Only walk once in each direction and minimize x = ∑  i p i - maximum of n steps required!  (  ) = 1/2 [ ∑  i p i ] T A ∑  i p i - b T [∑  i p i ] = 1/2 ∑ ∑  i  j p i T A p j - b T [∑  i p i ] (summations 0->n-1) n-1 i=0

Conjugate Gradients Method p i T Ap j =0 when i≠j. Such basis is mutually conjugate w/r to A. (p i are said to be ‘A orthogonal’)  (  ) = 1/2 ∑ ∑  i  j p i T A p j - b T [∑  i p i ] = 1/2 ∑  i  p i T A p i - b T [∑  i p i ] = 1/2 (∑  i  p i T A p i - 2  i b T p i ) - n independent terms Thus min  (  ) by minimizing i th term  i  p i T A p i - 2  i b T p i ->diff w/r  i and set derivative to zero:  i = b T p i / p i T A p i i.e., IF we have mutually conjugate basis, it is easy to minimize  (  )

Conjugate Gradients Method CG constructs sequence of x i, r i =b-Ax i, p i Start: x 0 =0, r 0 =b, p 0 =r 0,  0 =r 0 T r 0 /p 0 T Ap 0 Assume at k th iteration, we have x 0, x 1, …, x k ; r 0, r 1, …, r k ; p 0, p 1, …, p k ;  0,  1, …,  k Assume first k+1 basis vectors p i are mutually conjugate to A, first k+1 r i are mutually orthogonal, and r i T p j =0 for i ≠j Let x k+1 = x k +  k p k and r k+1 = r k -  k Ap k which updates correctly, since: r k+1 = b - Ax k+1 = b - A(x k +  k p k ) = (b-Ax k ) -  k Ap k = r k -  k Ap k

Conjugate Gradients Method Let x k+1 = x k +  k p k and r k+1 = r k -  k Ap k  k+1 = r k+1 T r k+1 /r k T r k p k+1 = r k+1 +  k+1 p k b T p k =r k T r k (eq 6.33-6.38) Now we need proof of the assumptions 1)r k+1 is orthogonal to r i for i≤k (eq 6.39-6.43) 2)r k+1 T r k =0 (eq 6.44-6.48) 3)r k+1 is orthogonal to p i for i≤k (eq 6.49-6.54) 4)p k+1 T Ap i = 0 for i≤k (eq 6.55-6.60) 5)i=k: p k+1 T Ap k = 0 ie CG generates mutually conjugate basis

Conjugate Gradients Method Thus shown that CG generates a sequence of mutually conjugate basis vectors. In theory, the method will find an exact solution in n iterations. Given positive definite, symmetric system of eqs Ax=b, initial solution x 0, let  0 =0, p -1 =0,r 0 =b-Ax 0, k=0 1.If k>0, let  k = r k T r k /r k-1 T r k-1 2.Let p k = r k +  k p k-1 3.Let  k = r k T r k / p k T A p k 4. Let x k+1 = x k +  k p k 5. Let r k+1 = r k -  k Ap k 6. Let k=k+1

Conjugate Gradients Least Squares Method CG can only be applied to positive definite systems of equations, thus not applicable to general LS problems. Instead, we can apply the CGLS method to min ||Gm-d|| 2 G T Gm=G T d

Conjugate Gradients Least Squares Method G T Gm=G T d r k = G T d-G T Gm k = G T (d-Gm k ) = G T s k s k+1 =d-Gm k+1 =d-G(m k +  k p k )=(d-Gm k )-  k Gp k =s k -  k Gp k Given a system of eqs Gm=d, k=0, m 0 =0, p -1 =0,  0 =0, r 0 =G T s 0. 1.If k>0, let  k = r k T r k /[r k-1 T r k-1 ] 2.Let p k = r k +  k p k-1 3.Let  k = r k T r k / [Gp k ] T [G p k ] 4. Let m k+1 = m k +  k p k 5. Let s k+1 =s k -  k Gp k 6. Let r k+1 = r k -  k Gp k 7. Let k=k+1; never computing G T G, only Gp k, G T s k+1

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