!求解方程組AX=B,其中A已經經過LU分解,結果存在數組B中
SUBROUTINE SOLVER(A,B,N)
IMPLICIT NONE
INTEGER N,I,K
DOUBLE PRECISION S,A(N,N),B(N) 
DO I=2,N
  S=0
  DO K=1,I-1
    S=S+A(I,K)*B(K)
  END DO
  B(I)=B(I)-S
END DO
B(N)=B(N)/A(N,N)
DO I=N-1,1,-1
  S=0
  DO K=I+1,N
    S=S+A(I,K)*B(K)
  END DO
  B(I)=(B(I)-S)/A(I,I)
END DO
END SUBROUTINE SOLVER