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TRIDIB(N,EPS1,D,E,E2,LB,UB,M11,M,W,IND,IERR,RV4,RV5)

       SUBROUTINE TRIDIB(N,EPS1,D,E,E2,LB,UB,M11,M,W,IND,IERR,RV4,RV5)
 C
       INTEGER I,J,K,L,M,N,P,Q,R,S,II,M1,M2,M11,M22,TAG,IERR,ISTURM
       REAL D(N),E(N),E2(N),W(M),RV4(N),RV5(N)
       REAL U,V,LB,T1,T2,UB,XU,X0,X1,EPS1,TST1,TST2,EPSLON
       INTEGER IND(M)
 C
 C     THIS SUBROUTINE IS A TRANSLATION OF THE ALGOL PROCEDURE BISECT,
 C     NUM. MATH. 9, 386-393(1967) BY BARTH, MARTIN, AND WILKINSON.
 C     HANDBOOK FOR AUTO. COMP., VOL.II-LINEAR ALGEBRA, 249-256(1971).
 C
 C     THIS SUBROUTINE FINDS THOSE EIGENVALUES OF A TRIDIAGONAL
 C     SYMMETRIC MATRIX BETWEEN SPECIFIED BOUNDARY INDICES,
 C     USING BISECTION.
 C
 C     ON INPUT
 C
 C        N IS THE ORDER OF THE MATRIX.
 C
 C        EPS1 IS AN ABSOLUTE ERROR TOLERANCE FOR THE COMPUTED
 C          EIGENVALUES.  IF THE INPUT EPS1 IS NON-POSITIVE,
 C          IT IS RESET FOR EACH SUBMATRIX TO A DEFAULT VALUE,
 C          NAMELY, MINUS THE PRODUCT OF THE RELATIVE MACHINE
 C          PRECISION AND THE 1-NORM OF THE SUBMATRIX.
 C
 C        D CONTAINS THE DIAGONAL ELEMENTS OF THE INPUT MATRIX.
 C
 C        E CONTAINS THE SUBDIAGONAL ELEMENTS OF THE INPUT MATRIX
 C          IN ITS LAST N-1 POSITIONS.  E(1) IS ARBITRARY.
 C
 C        E2 CONTAINS THE SQUARES OF THE CORRESPONDING ELEMENTS OF E.
 C          E2(1) IS ARBITRARY.
 C
 C        M11 SPECIFIES THE LOWER BOUNDARY INDEX FOR THE DESIRED
 C          EIGENVALUES.
 C
 C        M SPECIFIES THE NUMBER OF EIGENVALUES DESIRED.  THE UPPER
 C          BOUNDARY INDEX M22 IS THEN OBTAINED AS M22=M11+M-1.
 C
 C     ON OUTPUT
 C
 C        EPS1 IS UNALTERED UNLESS IT HAS BEEN RESET TO ITS
 C          (LAST) DEFAULT VALUE.
 C
 C        D AND E ARE UNALTERED.
 C
 C        ELEMENTS OF E2, CORRESPONDING TO ELEMENTS OF E REGARDED
 C          AS NEGLIGIBLE, HAVE BEEN REPLACED BY ZERO CAUSING THE
 C          MATRIX TO SPLIT INTO A DIRECT SUM OF SUBMATRICES.
 C          E2(1) IS ALSO SET TO ZERO.
 C
 C        LB AND UB DEFINE AN INTERVAL CONTAINING EXACTLY THE DESIRED
 C          EIGENVALUES.
 C
 C        W CONTAINS, IN ITS FIRST M POSITIONS, THE EIGENVALUES
 C          BETWEEN INDICES M11 AND M22 IN ASCENDING ORDER.
 C
 C        IND CONTAINS IN ITS FIRST M POSITIONS THE SUBMATRIX INDICES
 C          ASSOCIATED WITH THE CORRESPONDING EIGENVALUES IN W --
 C          1 FOR EIGENVALUES BELONGING TO THE FIRST SUBMATRIX FROM
 C          THE TOP, 2 FOR THOSE BELONGING TO THE SECOND SUBMATRIX, ETC..
 C
 C        IERR IS SET TO
 C          ZERO       FOR NORMAL RETURN,
 C          3*N+1      IF MULTIPLE EIGENVALUES AT INDEX M11 MAKE
 C                     UNIQUE SELECTION IMPOSSIBLE,
 C          3*N+2      IF MULTIPLE EIGENVALUES AT INDEX M22 MAKE
 C                     UNIQUE SELECTION IMPOSSIBLE.
 C
 C        RV4 AND RV5 ARE TEMPORARY STORAGE ARRAYS.
 C
 C     NOTE THAT SUBROUTINE TQL1, IMTQL1, OR TQLRAT IS GENERALLY FASTER
 C     THAN TRIDIB, IF MORE THAN N/4 EIGENVALUES ARE TO BE FOUND.
 C
 C     QUESTIONS AND COMMENTS SHOULD BE DIRECTED TO BURTON S. GARBOW,
 C     MATHEMATICS AND COMPUTER SCIENCE DIV, ARGONNE NATIONAL LABORATORY
 C
 C     THIS VERSION DATED AUGUST 1983.
 C
 C     ------------------------------------------------------------------
 C