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146 lines
4.4 KiB
Fortran
146 lines
4.4 KiB
Fortran
*DECK DTRSL
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SUBROUTINE DTRSL (T, LDT, N, B, JOB, INFO)
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C***BEGIN PROLOGUE DTRSL
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C***PURPOSE Solve a system of the form T*X=B or TRANS(T)*X=B, where
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C T is a triangular matrix.
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C***LIBRARY SLATEC (LINPACK)
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C***CATEGORY D2A3
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C***TYPE DOUBLE PRECISION (STRSL-S, DTRSL-D, CTRSL-C)
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C***KEYWORDS LINEAR ALGEBRA, LINPACK, TRIANGULAR LINEAR SYSTEM,
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C TRIANGULAR MATRIX
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C***AUTHOR Stewart, G. W., (U. of Maryland)
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C***DESCRIPTION
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C
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C DTRSL solves systems of the form
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C
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C T * X = B
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C or
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C TRANS(T) * X = B
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C
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C where T is a triangular matrix of order N. Here TRANS(T)
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C denotes the transpose of the matrix T.
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C
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C On Entry
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C
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C T DOUBLE PRECISION(LDT,N)
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C T contains the matrix of the system. The zero
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C elements of the matrix are not referenced, and
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C the corresponding elements of the array can be
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C used to store other information.
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C
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C LDT INTEGER
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C LDT is the leading dimension of the array T.
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C
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C N INTEGER
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C N is the order of the system.
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C
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C B DOUBLE PRECISION(N).
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C B contains the right hand side of the system.
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C
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C JOB INTEGER
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C JOB specifies what kind of system is to be solved.
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C If JOB is
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C
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C 00 solve T*X=B, T lower triangular,
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C 01 solve T*X=B, T upper triangular,
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C 10 solve TRANS(T)*X=B, T lower triangular,
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C 11 solve TRANS(T)*X=B, T upper triangular.
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C
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C On Return
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C
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C B B contains the solution, if INFO .EQ. 0.
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C Otherwise B is unaltered.
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C
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C INFO INTEGER
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C INFO contains zero if the system is nonsingular.
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C Otherwise INFO contains the index of
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C the first zero diagonal element of T.
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C
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C***REFERENCES J. J. Dongarra, J. R. Bunch, C. B. Moler, and G. W.
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C Stewart, LINPACK Users' Guide, SIAM, 1979.
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C***ROUTINES CALLED DAXPY, DDOT
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C***REVISION HISTORY (YYMMDD)
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C 780814 DATE WRITTEN
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C 890831 Modified array declarations. (WRB)
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C 890831 REVISION DATE from Version 3.2
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 900326 Removed duplicate information from DESCRIPTION section.
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C (WRB)
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C 920501 Reformatted the REFERENCES section. (WRB)
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C***END PROLOGUE DTRSL
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INTEGER LDT,N,JOB,INFO
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DOUBLE PRECISION T(LDT,*),B(*)
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C
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C
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DOUBLE PRECISION DDOT,TEMP
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INTEGER CASE,J,JJ
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C***FIRST EXECUTABLE STATEMENT DTRSL
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C
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C CHECK FOR ZERO DIAGONAL ELEMENTS.
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C
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DO 10 INFO = 1, N
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IF (T(INFO,INFO) .EQ. 0.0D0) GO TO 150
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10 CONTINUE
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INFO = 0
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C
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C DETERMINE THE TASK AND GO TO IT.
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C
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CASE = 1
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IF (MOD(JOB,10) .NE. 0) CASE = 2
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IF (MOD(JOB,100)/10 .NE. 0) CASE = CASE + 2
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GO TO (20,50,80,110), CASE
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C
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C SOLVE T*X=B FOR T LOWER TRIANGULAR
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C
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20 CONTINUE
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B(1) = B(1)/T(1,1)
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IF (N .LT. 2) GO TO 40
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DO 30 J = 2, N
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TEMP = -B(J-1)
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CALL DAXPY(N-J+1,TEMP,T(J,J-1),1,B(J),1)
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B(J) = B(J)/T(J,J)
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30 CONTINUE
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40 CONTINUE
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GO TO 140
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C
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C SOLVE T*X=B FOR T UPPER TRIANGULAR.
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C
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50 CONTINUE
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B(N) = B(N)/T(N,N)
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IF (N .LT. 2) GO TO 70
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DO 60 JJ = 2, N
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J = N - JJ + 1
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TEMP = -B(J+1)
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CALL DAXPY(J,TEMP,T(1,J+1),1,B(1),1)
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B(J) = B(J)/T(J,J)
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60 CONTINUE
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70 CONTINUE
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GO TO 140
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C
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C SOLVE TRANS(T)*X=B FOR T LOWER TRIANGULAR.
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C
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80 CONTINUE
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B(N) = B(N)/T(N,N)
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IF (N .LT. 2) GO TO 100
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DO 90 JJ = 2, N
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J = N - JJ + 1
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B(J) = B(J) - DDOT(JJ-1,T(J+1,J),1,B(J+1),1)
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B(J) = B(J)/T(J,J)
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90 CONTINUE
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100 CONTINUE
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GO TO 140
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C
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C SOLVE TRANS(T)*X=B FOR T UPPER TRIANGULAR.
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C
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110 CONTINUE
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B(1) = B(1)/T(1,1)
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IF (N .LT. 2) GO TO 130
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DO 120 J = 2, N
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B(J) = B(J) - DDOT(J-1,T(1,J),1,B(1),1)
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B(J) = B(J)/T(J,J)
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120 CONTINUE
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130 CONTINUE
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140 CONTINUE
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150 CONTINUE
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RETURN
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END
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