bnmatrxa.h

ARPACK is a collection of Fortran77 subroutines designed to solve large scale eigenvalue problems.

/*
   ARPACK++ v1.0 8/1/1997
   c++ interface to ARPACK code.

   MODULE BNMatrxA.h
   Function template for the matrix

                          | T -I          |
                          |-I  T -I       |
                      A = |   -I  T       |
                          |        ...  -I|
                          |           -I T|

   derived from the standard central difference discretization of the
   2-dimensional convection-diffusion operator (Laplacian u) + rho*(du/dx)
   on a unit square with zero Dirichlet boundary conditions.
   When rho*h/2 <= 1, the discrete convection-diffusion operator has real
   eigenvalues.  When rho*h/2 > 1, it has COMPLEX eigenvalues.

   ARPACK Authors
      Richard Lehoucq
      Danny Sorensen
      Chao Yang
      Dept. of Computational & Applied Mathematics
      Rice University
      Houston, Texas
*/

#ifndef BNMATRXA_H
#define BNMATRXA_H

#include <math.h>

template<class FLOAT, class INT>
void BandMatrixA(INT nx, FLOAT rho, INT& n, INT& nL, INT& nU, FLOAT* &A)

{

  // Defining internal variables.

  INT    i, j, lda;
  FLOAT  h, h2, df;
  FLOAT  dd, dl, du;

  // Defining constants.

  h   = 1.0/FLOAT(nx+1);
  h2  = h*h;
  dd  = 4.0/h2;
  df  = -1.0/h2;
  dl  = df - 5.0e-1*rho/h;
  du  = df + 5.0e-1*rho/h;

  // Defining the number of columns and the upper and lower bandwidth.

  n   = nx*nx;
  nL  = nx;
  nU  = nx;

  // Creating output vector A.

  lda = nL+nU+1;
  A   = new FLOAT[n*lda];

  // Filling A with zeros.

  for (j=0; j<(n*lda); j++ ) A[j] = (FLOAT)0.0;

  // Creating matrix A.

  for (i=0, j=nU; i<n; i++, j+=lda) {
    if (i>=nx)     A[j-nx] = df;
    if (i%nx)      A[j-1]  = du;
                   A[j]    = dd;
    if ((i+1)%nx)  A[j+1]  = dl;
    if (i<(n-nx))  A[j+nx] = df;
  }
    
} // BandMatrixA.

#endif // BNMATRXA_H