Category : Miscellaneous Language Source Code
Archive   : FORTRN77.ZIP
Filename : NAREGU.FOR
C
C THIS SUBROUTINE FINDS THE ROOT OF THE EQUATION F(X) = 0
C THROUGH THE METHOD OF FALSE POSITION(REGULA FALSI).
C
NMAX = N
N = 0
CALL VALUE(XHI,FXHI)
CALL VALUE(XLOW,FXLOW)
IF(FXHI.GE.0.0) GO TO 20
XP = XLOW
FXP = FXLOW
XQ = XHI
FXQ = FXHI
GO TO 7
20 XP = XHI
FXP = FXHI
XQ = XLOW
FXQ = FXLOW
7 XNEW = XP - FXP*(XP - XQ)/(FXP - FXQ)
CALL VALUE(XNEW,FXNEW)
N = N + 1
IF(ABS(FXNEW).GT.EPS) GO TO 2
ROOT = XNEW
N = 1
RETURN
2 IF(N.LT.NMAX) GO TO 40
ROOT = XNEW
N = 2
RETURN
40 IF(FXNEW.GT.0.0) GO TO 4
X = XNEW - FXNEW*(XNEW-XQ)/(FXNEW - FXQ)
XQ = XNEW
FXQ = FXNEW
15 IF(XP.LE.XQ) GO TO 6
IF(XP.LE.X) GO TO 7
IF(X.LE.XQ) GO TO 7
9 N = N + 1
CALL VALUE(X,FX)
IF(ABS(FX).GT.EPS) GO TO 17
ROOT = X
N = 1
RETURN
17 IF(N.LT.NMAX) GO TO 18
ROOT = X
N = 2
RETURN
18 IF(FX.GT.0.0) GO TO 11
XQ = X
FXQ = FX
GO TO 7
11 XP = X
FXP = FX
GO TO 7
6 IF(XQ.LE.X) GO TO 7
IF(X.LE.XP) GO TO 7
GO TO 9
4 X = XNEW - FXNEW*(XNEW - XP)/(FXNEW - FXP)
XP = XNEW
FXP = FXNEW
GO TO 15
RETURN
END
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