[数值问题] FORTRAN里TOL怎么定义，为什么我的计算不准

program test_epsilon
    real :: x = 3.143
    real(8) :: y = 2.33
    print *, EPSILON(x)
    print *, EPSILON(y)
end program test_epsilon

	SUBROUTINE DOOLITTLE(A,L,U,N)
		!Doolittle分解
		IMPLICIT REAL*8(A-Z)
		INTEGER::N,I,K,R
		REAL*8::A(N,N),L(N,N),U(N,N)
        N = 501
		!U的第一行
		U(1,:)=A(1,:)
		!L的第一列
		L(:,1)=A(:,1)/U(1,1)
		DO K=2,N
			L(K,K)=1
			DO J=K,N
				S=0
				DO M=1,K-1
					S=S+L(K,M)*U(M,J)
				END DO
				U(K,J)=A(K,J)-S
			END DO
			DO I=K+1,N
				S=0
				DO M=1,K-1
					S=S+L(I,M)*U(M,K)
				END DO
				L(I,K)=(A(I,K)-S)/U(K,K)
			END DO
		END DO
	END SUBROUTINE DOOLITTLE
	SUBROUTINE UPTRI(A,B,X,N)
	!解上三角方程组
		IMPLICIT REAL*8(A-Z)
		INTEGER::I,J,K,N
		REAL*8::A(N,N),B(N),X(N)
        N = 501
		X(N)=B(N)/A(N,N)
		!回代部分
		DO I=N-1,1,-1
			X(I)=B(I)
			DO J=I+1,N
				X(I)=X(I)-A(I,J)*X(J)
			END DO
			X(I)=X(I)/A(I,I)
		END DO
	END SUBROUTINE UPTRI
	SUBROUTINE DOWNTRI(A,B,X,N)
	!解下三角方程组
		IMPLICIT REAL*8(A-Z)
		INTEGER::I,J,N
		REAL*8::A(N,N),B(N),X(N)
        N = 501
		X(1)=B(1)/A(1,1)
		DO K=2,N
			X(K)=B(K)
			DO I=1,K-1
				X(K)=X(K)-A(K,I)*X(I)
			END DO
			X(K)=X(K)/A(K,K)
		END DO
    END SUBROUTINE DOWNTRI
    SUBROUTINE UNMIFA(A,NAMDA4)
     IMPLICIT NONE
		INTEGER::N,I,K,L,S,J,M,K1
		DIMENSION A(501,501),U(501),U0(501),V(501),U1(501,501),L1(501,501),Y(501)
        DOUBLE PRECISION MA,A,NAMDA4,U,U0,V,M0,M1,TOL,U1,L1,Y
        N = 501
    	DO I=1,N
			U0(N)=10D0
        END DO
        !给出u0
		U=U0
		M0=0
        !------------------------------------------------
        CALL DOOLITTLE(A,L1,U1,N)
        !------------------------------------------------
		DO K1=1,500
        CALL DOWNTRI(L1,U,Y,N)
		CALL UPTRI(U1,Y,V,N)
        !------------------------------------------------
            !用解方程组的方法求矩阵v
            !求最大值
            M1=DABS(V(1))
        DO I=2,N
			IF (DABS(V(I))>M1) THEN
				M1=DABS(V(I))
				L = I !用k记录指标，但是ma不取r(i)
			END IF
        END DO
		M1 = V(L)
            !找出v中的最大值，返回到m1 SUBROUTINE MAX_ROU(R,N,MA)
			U = V/M1
			!IF (DABS(M1-M0)<1E-20) EXIT
            IF (DABS(DABS(M1)-DABS(M0))/DABS(M1)<TOL) EXIT
				M0=M1
		END DO
		NAMDA4=M1
        NAMDA4=1/NAMDA4
    END  
    PROGRAM MAIN
    IMPLICIT NONE
    INTEGER I,J,N
    DIMENSION A(501,501)
    DOUBLE PRECISION A,NAMDA4
    !给a赋值!!!!!!!!!!!!!!!!!!!!!!!!!
     DO I = 1,501
        DO J = 1,501
        IF(I == J) THEN 
            A(I,J) =(1.64-0.024*I)*SIN(0.2*I)-0.64*EXP(0.1/I)
        ELSE IF(I == J+1) THEN 
            A(I,J) = 0.16
        ELSE IF(I+1 == J) THEN
            A(I,J) = 0.16
        ELSE IF(I == J+2) THEN 
            A(I,J) = -0.064
        ELSE IF(I+2 == J) THEN 
            A(I,J) = -0.064
        !ELSE 
          ! A(I,J) = 0
            END IF
        END DO
     END DO
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
     CALL UNMIFA(A,NAMDA4)
     WRITE(*,*) NAMDA4
    END