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| 1 | SUBROUTINE DROTG(DA,DB,C,S) |
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| 2 | * .. Scalar Arguments .. |
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| 3 | DOUBLE PRECISION C,DA,DB,S |
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| 4 | * .. |
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| 5 | * |
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| 6 | * Purpose |
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| 7 | * ======= |
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| 8 | * |
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| 9 | * construct givens plane rotation. |
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| 10 | * jack dongarra, linpack, 3/11/78. |
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| 11 | * |
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| 12 | * |
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| 13 | * .. Local Scalars .. |
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| 14 | DOUBLE PRECISION R,ROE,SCALE,Z |
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| 15 | * .. |
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| 16 | * .. Intrinsic Functions .. |
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| 17 | INTRINSIC DABS,DSIGN,DSQRT |
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| 18 | * .. |
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| 19 | ROE = DB |
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| 20 | IF (DABS(DA).GT.DABS(DB)) ROE = DA |
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| 21 | SCALE = DABS(DA) + DABS(DB) |
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| 22 | IF (SCALE.NE.0.0d0) GO TO 10 |
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| 23 | C = 1.0d0 |
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| 24 | S = 0.0d0 |
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| 25 | R = 0.0d0 |
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| 26 | Z = 0.0d0 |
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| 27 | GO TO 20 |
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| 28 | 10 R = SCALE*DSQRT((DA/SCALE)**2+ (DB/SCALE)**2) |
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| 29 | R = DSIGN(1.0d0,ROE)*R |
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| 30 | C = DA/R |
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| 31 | S = DB/R |
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| 32 | Z = 1.0d0 |
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| 33 | IF (DABS(DA).GT.DABS(DB)) Z = S |
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| 34 | IF (DABS(DB).GE.DABS(DA) .AND. C.NE.0.0d0) Z = 1.0d0/C |
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| 35 | 20 DA = R |
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| 36 | DB = Z |
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| 37 | RETURN |
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| 38 | END |
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