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[u/mrichter/AliRoot.git] / MINICERN / mathlib / gen / e / slv.F

*
* $Id$
*
* $Log$
* Revision 1.1.1.1  1996/04/01 15:02:27  mclareni
* Mathlib gen
*
*
#include "gen/pilot.h"
      SUBROUTINE SLV  (M,N,QR,ALPHA,IPIVOT,R,Y,Z)
      DIMENSION  QR(M,N),ALPHA(N),R(M),Y(N),IPIVOT(N),Z(N)
C
C        USING THE VECTORS U, WHOSE NON-ZERO COMPONENTS ARE
C        STORED ON AND BELOW THE MAIN DIAGONAL OF QR(M,N), SLV
C        APPLIES THE N TRANSFORMATIONS (I - BETA UT) TO THE
C        RIGHT HAND SIDE R(M).  FROM THE REDUCED MATRIX GIVEN IN
C        ALPHA(N) AND THE UPPER RIGHT TRIANGULAR PART OF QR,
C        SLV THEN COMPUTES BY BACK-SUBSTITUTION, AN APPROXIMATE
C        SOLUTION TO THE LINEAR SYSTEM.  THE COMPONENTS OF THE
C        SOLUTION VECTOR ARE STORED IN Y(N) IN THE ORDER PRESCRIBED
C        BY IPIVOT(N).
C
      DO 10  J=1,N
      GAMMA=PROD1(QR(1,J),R,J,M)/(ALPHA(J)*QR(J,J))
      DO 5  I=J,M
 5    R(I)=R(I)+GAMMA*QR(I,J)
 10   CONTINUE
      Z(N)=R(N)/ALPHA(N)
      NIT=N-1
      DO 15  I=1,NIT
      L=N-I
      LA=L+1
 15   Z(L)=-PROD2(QR,Z,M,LA,N,L,-R(L))/ALPHA(L)
      DO 20  I=1,N
      JU=IPIVOT(I)
 20   Y(JU)=Z(I)
      RETURN
      END