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57 lines
1.6 KiB
Fortran
57 lines
1.6 KiB
Fortran
*DECK TRIS4
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SUBROUTINE TRIS4 (N, A, B, C, D, U, Z)
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C***BEGIN PROLOGUE TRIS4
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C***SUBSIDIARY
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C***PURPOSE Subsidiary to SEPX4
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C***LIBRARY SLATEC
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C***TYPE SINGLE PRECISION (TRIS4-S)
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C***AUTHOR (UNKNOWN)
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C***DESCRIPTION
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C
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C This subroutine solves for a non-zero eigenvector corresponding
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C to the zero eigenvalue of the transpose of the rank
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C deficient ONE matrix with subdiagonal A, diagonal B, and
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C superdiagonal C , with A(1) in the (1,N) position, with
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C C(N) in the (N,1) position, AND all other elements zero.
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C
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C***SEE ALSO SEPX4
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C***ROUTINES CALLED (NONE)
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C***REVISION HISTORY (YYMMDD)
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C 801001 DATE WRITTEN
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C 890831 Modified array declarations. (WRB)
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 900402 Added TYPE section. (WRB)
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C***END PROLOGUE TRIS4
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C
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DIMENSION A(*) ,B(*) ,C(*) ,D(*) ,
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1 U(*) ,Z(*)
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C***FIRST EXECUTABLE STATEMENT TRIS4
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BN = B(N)
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D(1) = A(2)/B(1)
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V = A(1)
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U(1) = C(N)/B(1)
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NM2 = N-2
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DO 10 J=2,NM2
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DEN = B(J)-C(J-1)*D(J-1)
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D(J) = A(J+1)/DEN
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U(J) = -C(J-1)*U(J-1)/DEN
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BN = BN-V*U(J-1)
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V = -V*D(J-1)
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10 CONTINUE
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DEN = B(N-1)-C(N-2)*D(N-2)
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D(N-1) = (A(N)-C(N-2)*U(N-2))/DEN
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AN = C(N-1)-V*D(N-2)
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BN = BN-V*U(N-2)
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DEN = BN-AN*D(N-1)
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C
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C SET LAST COMPONENT EQUAL TO ONE
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C
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Z(N) = 1.0
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Z(N-1) = -D(N-1)
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NM1 = N-1
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DO 20 J=2,NM1
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K = N-J
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Z(K) = -D(K)*Z(K+1)-U(K)*Z(N)
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20 CONTINUE
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RETURN
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END
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