Newton-Raphson.f90

PROGRAM ME
	EXTERNAL ff,ffder
	x0=1
	CALL newton(ff,ffder,x0,1e-7,1e-7,10,iflag)
	IF(iflag>1) THEN
		PRINT *, "there was an error occurred"
	ENDIF
	fx0=ff(x0)
	PRINT *, x0,fx0
	READ (*,*)
END PROGRAM ME

SUBROUTINE newton(F,Fder,X0,Xtol,Ftol,Ntol,iflag)
	PRINT *, "          N      x0              F(x0)            der(F)"
	iflag=0
	DO N=1,Ntol
		Fx0=F(x0)
		IF(abs(Fx0)<ftol) THEN
			RETURN
		ENDIF
		derF=Fder(x0)
		PRINT *, N,x0,Fx0,derF
		IF(derF==0)THEN
			PRINT *, "error, derivative=0"
			iflag=2
			RETURN
		ENDIF
		DELTAx=Fx0/derF
		x0=x0-DELTAx
		IF(abs(DELTAx)<xtol)THEN
			RETURN
		ENDIF
	ENDDO
END SUBROUTINE newton

FUNCTION ff(x)
	ff=x*x*x-x-1
	RETURN
END FUNCTION

FUNCTION ffder(x)
	ffder=3*x*x-1
	RETURN
END FUNCTION ffder