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SUBROUTINE RWUPDT (N, R, LDR, W, B, ALPHA, COS, SIN)
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C***BEGIN PROLOGUE RWUPDT
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C***PURPOSE Subsidiary to SNLS1 and SNLS1E
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C***TYPE SINGLE PRECISION (RWUPDT-S, DWUPDT-D)
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C Given an N by N upper triangular matrix R, this subroutine
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C computes the QR decomposition of the matrix formed when a row
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C is added to R. If the row is specified by the vector W, then
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C RWUPDT determines an orthogonal matrix Q such that when the
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C N+1 by N matrix composed of R augmented by W is premultiplied
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C by (Q TRANSPOSE), the resulting matrix is upper trapezoidal.
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C The orthogonal matrix Q is the product of N transformations
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C G(1)*G(2)* ... *G(N)
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C where G(I) is a Givens rotation in the (I,N+1) plane which
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C eliminates elements in the I-th plane. RWUPDT also
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C computes the product (Q TRANSPOSE)*C where C is the
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C (N+1)-vector (b,alpha). Q itself is not accumulated, rather
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C the information to recover the G rotations is supplied.
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C The subroutine statement is
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C SUBROUTINE RWUPDT(N,R,LDR,W,B,ALPHA,COS,SIN)
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C N is a positive integer input variable set to the order of R.
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C R is an N by N array. On input the upper triangular part of
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C R must contain the matrix to be updated. On output R
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C contains the updated triangular matrix.
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C LDR is a positive integer input variable not less than N
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C which specifies the leading dimension of the array R.
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C W is an input array of length N which must contain the row
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C vector to be added to R.
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C B is an array of length N. On input B must contain the
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C first N elements of the vector C. On output B contains
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C the first N elements of the vector (Q TRANSPOSE)*C.
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C ALPHA is a variable. On input ALPHA must contain the
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C (N+1)-st element of the vector C. On output ALPHA contains
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C the (N+1)-st element of the vector (Q TRANSPOSE)*C.
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C COS is an output array of length N which contains the
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C cosines of the transforming Givens rotations.
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C SIN is an output array of length N which contains the
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C sines of the transforming Givens rotations.
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C***SEE ALSO SNLS1, SNLS1E
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C***ROUTINES CALLED (NONE)
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C***REVISION HISTORY (YYMMDD)
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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 900326 Removed duplicate information from DESCRIPTION section.
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C 900328 Added TYPE section. (WRB)
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C***END PROLOGUE RWUPDT
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REAL R(LDR,*),W(*),B(*),COS(*),SIN(*)
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REAL COTAN,ONE,P5,P25,ROWJ,TAN,TEMP,ZERO
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SAVE ONE, P5, P25, ZERO
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DATA ONE,P5,P25,ZERO /1.0E0,5.0E-1,2.5E-1,0.0E0/
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C***FIRST EXECUTABLE STATEMENT RWUPDT
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C APPLY THE PREVIOUS TRANSFORMATIONS TO
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C R(I,J), I=1,2,...,J-1, AND TO W(J).
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IF (JM1 .LT. 1) GO TO 20
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TEMP = COS(I)*R(I,J) + SIN(I)*ROWJ
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ROWJ = -SIN(I)*R(I,J) + COS(I)*ROWJ
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C DETERMINE A GIVENS ROTATION WHICH ELIMINATES W(J).
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IF (ROWJ .EQ. ZERO) GO TO 50
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IF (ABS(R(J,J)) .GE. ABS(ROWJ)) GO TO 30
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SIN(J) = P5/SQRT(P25+P25*COTAN**2)
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COS(J) = SIN(J)*COTAN
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COS(J) = P5/SQRT(P25+P25*TAN**2)
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C APPLY THE CURRENT TRANSFORMATION TO R(J,J), B(J), AND ALPHA.
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R(J,J) = COS(J)*R(J,J) + SIN(J)*ROWJ
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TEMP = COS(J)*B(J) + SIN(J)*ALPHA
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ALPHA = -SIN(J)*B(J) + COS(J)*ALPHA
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C LAST CARD OF SUBROUTINE RWUPDT.
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src/source_f/slatec/rwupdt.f
