pminres(4rheolef)						    rheolef-6.1 						 pminres(4rheolef)

NAME

       pminres -- conjugate gradient algorithm.

SYNOPSIS

	   template <class Matrix, class Vector, class Preconditioner, class Real>
	   int pminres (const Matrix &A, Vector &x, const Vector &b, const Preconditioner &M,
	     int &max_iter, Real &tol, odiststream *p_derr=0);

EXAMPLE

       The simplest call to 'pminres' has the folling form:

	   size_t max_iter = 100;
	   double tol = 1e-7;
	   int status = pminres(a, x, b, EYE, max_iter, tol, &derr);

DESCRIPTION

       pminres solves the symmetric positive definite linear system Ax=b using the Conjugate Gradient method.

       The return value indicates convergence within max_iter (input) iterations(0), or no convergence within max_iter iterations(1).  Upon suc-
       cessful return, output arguments have the following values:

       x      approximate solution to Ax = b

       max_iter
	      the number of iterations performed before the tolerance was reached

       tol    the residual after the final iteration

NOTE

       pminres follows the algorithm described in "Solution of sparse indefinite systems of linear equations", C. C. Paige  and  M.  A.  Saunders,
       SIAM  J.  Numer. Anal., 12(4), 1975.  For more, see http://www.stanford.edu/group/SOL/software.html and also the PhD "Iterative methods for
       singular linear equations and least-squares problems", S.-C. T. Choi, Stanford University, 2006,  http://www.stanford.edu/group/SOL/disser-
       tations/sou-cheng-choi-thesis.pdf  at  page  60.   The  present	implementation	style is inspired from IML++ 1.2 iterative method library,
       http://math.nist.gov/iml++.

IMPLEMENTATION

       template <class Matrix, class Vector, class Preconditioner, class Real, class Size>
       int pminres(const Matrix &A, Vector &x, const Vector &Mb, const Preconditioner &M,
	 Size &max_iter, Real &tol, odiststream *p_derr = 0, std::string label = "minres")
       {
	 Vector b = M.solve(Mb);
	 Real norm_b = sqrt(fabs(dot(Mb,b)));
	 if (norm_b == Real(0.)) norm_b = 1;

	 Vector Mr = Mb - A*x;
	 Vector z = M.solve(Mr);
	 Real beta2 = dot(Mr, z);
	 Real norm_r = sqrt(fabs(beta2));
	 if (p_derr) (*p_derr) << "[" << label << "] #iteration residue" << std::endl;
	 if (p_derr) (*p_derr) << "[" << label << "] 0 " << norm_r/norm_b << std::endl;
	 if (beta2 < 0 || norm_r <= tol*norm_b) {
	   tol = norm_r/norm_b;
	   max_iter = 0;
	   dis_warning_macro ("beta2 = " << beta2 << " < 0: stop");
	   return 0;
	 }
	 Real beta = sqrt(beta2);
	 Real eta = beta;
	 Vector Mv = Mr/beta;
	 Vector  u =  z/beta;
	 Real c_old = 1.;
	 Real s_old = 0.;
	 Real c = 1.;
	 Real s = 0.;
	 Vector u_old  (x.ownership(), 0.);
	 Vector Mv_old (x.ownership(), 0.);
	 Vector w      (x.ownership(), 0.);
	 Vector w_old  (x.ownership(), 0.);
	 Vector w_old2 (x.ownership(), 0.);
	 for (Size n = 1; n <= max_iter; n++) {
	   // Lanczos
	   Mr = A*u;
	   z = M.solve(Mr);
	   Real alpha = dot(Mr, u);
	   Mr = Mr - alpha*Mv - beta*Mv_old;
	   z  =  z - alpha*u  - beta*u_old;
	   beta2 = dot(Mr, z);
	   if (beta2 < 0) {
	       dis_warning_macro ("pminres: machine precision problem");
	       tol = norm_r/norm_b;
	       max_iter = n;
	       return 2;
	   }
	   Real beta_old = beta;
	   beta = sqrt(beta2);
	   // QR factorisation
	   Real c_old2 = c_old;
	   Real s_old2 = s_old;
	   c_old = c;
	   s_old = s;
	   Real rho0 = c_old*alpha - c_old2*s_old*beta_old;
	   Real rho2 = s_old*alpha + c_old2*c_old*beta_old;
	   Real rho1 = sqrt(sqr(rho0) + sqr(beta));
	   Real rho3 = s_old2 * beta_old;
	   // Givens rotation
	   c = rho0 / rho1;
	   s = beta / rho1;
	   // update
	   w_old2 = w_old;
	   w_old  = w;
	   w = (u - rho2*w_old - rho3*w_old2)/rho1;
	   x += c*eta*w;
	   eta = -s*eta;
	   Mv_old = Mv;
	    u_old = u;
	   Mv = Mr/beta;
	    u =  z/beta;
	   // check residue
	   norm_r *= s;
	   if (p_derr) (*p_derr) << "[" << label << "] " << n << " " << norm_r/norm_b << std::endl;
	   if (norm_r <= tol*norm_b) {
	     tol = norm_r/norm_b;
	     max_iter = n;
	     return 0;
	   }
	 }
	 tol = norm_r/norm_b;
	 return 1;
       }

rheolef-6.1							    rheolef-6.1 						 pminres(4rheolef)