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ElmerCSC
GitHub Repository: ElmerCSC/elmerfem
 Path: blob/devel/mathlibs/src/arpack/dnconv.f
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c-----------------------------------------------------------------------

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c\BeginDoc

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c

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c\Name: dnconv

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c

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c\Description: 

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c  Convergence testing for the nonsymmetric Arnoldi eigenvalue routine.

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c

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c\Usage:

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c  call dnconv

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c     ( N, RITZR, RITZI, BOUNDS, TOL, NCONV )

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c

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c\Arguments

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c  N       Integer.  (INPUT)

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c          Number of Ritz values to check for convergence.

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c

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c  RITZR,  Double precision arrays of length N.  (INPUT)

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c  RITZI   Real and imaginary parts of the Ritz values to be checked

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c          for convergence.

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c  BOUNDS  Double precision array of length N.  (INPUT)

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c          Ritz estimates for the Ritz values in RITZR and RITZI.

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c

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c  TOL     Double precision scalar.  (INPUT)

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c          Desired backward error for a Ritz value to be considered

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c          "converged".

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c

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c  NCONV   Integer scalar.  (OUTPUT)

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c          Number of "converged" Ritz values.

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c

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c\EndDoc

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c

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c-----------------------------------------------------------------------

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c

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c\BeginLib

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c

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c\Local variables:

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c     xxxxxx  real

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c

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c\Routines called:

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c     second  ARPACK utility routine for timing.

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c     dlamch  LAPACK routine that determines machine constants.

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c     dlapy2  LAPACK routine to compute sqrt(x**2+y**2) carefully.

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c

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c\Author

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c     Danny Sorensen               Phuong Vu

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c     Richard Lehoucq              CRPC / Rice University 

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c     Dept. of Computational &     Houston, Texas

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c     Applied Mathematics 

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c     Rice University           

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c     Houston, Texas    

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c

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c\Revision history:

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c     xx/xx/92: Version ' 2.1'

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c

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c\SCCS Information: @(#) 

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c FILE: nconv.F   SID: 2.3   DATE OF SID: 4/20/96   RELEASE: 2

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c

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c\Remarks

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c     1. xxxx

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c

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c\EndLib

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c

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c-----------------------------------------------------------------------

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c

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      subroutine dnconv (n, ritzr, ritzi, bounds, tol, nconv)

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c

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c     %----------------------------------------------------%

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c     | Include files for debugging and timing information |

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c     %----------------------------------------------------%

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c

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      include   'debug.h'

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      include   'stat.h'

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c

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c     %------------------%

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c     | Scalar Arguments |

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c     %------------------%

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c

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      integer    n, nconv

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      Double precision

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     &           tol

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c

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c     %-----------------%

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c     | Array Arguments |

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c     %-----------------%

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      Double precision

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     &           ritzr(n), ritzi(n), bounds(n)

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c

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c     %---------------%

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c     | Local Scalars |

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c     %---------------%

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c

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      integer    i

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      Double precision

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     &           temp, eps23

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c

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c     %--------------------%

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c     | External Functions |

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c     %--------------------%

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c

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      Double precision

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     &           dlapy2, dlamch

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      external   dlapy2, dlamch

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c     %-----------------------%

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c     | Executable Statements |

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c     %-----------------------%

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c 

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c     %-------------------------------------------------------------%

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c     | Convergence test: unlike in the symmetric code, I am not    |

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c     | using things like refined error bounds and gap condition    |

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c     | because I don't know the exact equivalent concept.          |

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c     |                                                             |

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c     | Instead the i-th Ritz value is considered "converged" when: |

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c     |                                                             |

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c     |     bounds(i) .le. ( TOL * | ritz | )                       |

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c     |                                                             |

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c     | for some appropriate choice of norm.                        |

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c     %-------------------------------------------------------------%

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c

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      call second (t0)

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c

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c     %---------------------------------%

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c     | Get machine dependent constant. |

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c     %---------------------------------%

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c

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      eps23 = dlamch('Epsilon-Machine')

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      eps23 = eps23**(2.0D+0 / 3.0D+0)

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c

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      nconv  = 0

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      do 20 i = 1, n

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         temp = max( eps23, dlapy2( ritzr(i), ritzi(i) ) )

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         if (bounds(i) .le. tol*temp)   nconv = nconv + 1

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   20 continue

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c 

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      call second (t1)

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      tnconv = tnconv + (t1 - t0)

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c 

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      return

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c

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c     %---------------%

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c     | End of dnconv |

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c     %---------------%

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c

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      end

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