minverse3d.for

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      SUBROUTINE matinv3d(A,A_INV,NDI)
C>    INVERSE OF A 3X3 MATRIX
C     RETURN THE INVERSE OF A(3,3) - A_INV
      IMPLICIT NONE
C
      INTEGER NDI
C
      DOUBLE PRECISION A(ndi,ndi),A_INV(ndi,ndi),DET_A,DET_A_INV
C
      det_a = a(1,1)*(a(2,2)*a(3,3) - a(3,2)*a(2,3)) -
     +        a(2,1)*(a(1,2)*a(3,3) - a(3,2)*a(1,3)) +
     +        a(3,1)*(a(1,2)*a(2,3) - a(2,2)*a(1,3))

      IF (det_a .LE. 0.d0) THEN
        WRITE(*,*) 'WARNING: SUBROUTINE MATINV3D:'
        WRITE(*,*) 'WARNING: DET OF MAT=',det_a
        RETURN
      END IF
C
      det_a_inv = 1.d0/det_a
C
      a_inv(1,1) = det_a_inv*(a(2,2)*a(3,3)-a(3,2)*a(2,3))
      a_inv(1,2) = det_a_inv*(a(3,2)*a(1,3)-a(1,2)*a(3,3))
      a_inv(1,3) = det_a_inv*(a(1,2)*a(2,3)-a(2,2)*a(1,3))
      a_inv(2,1) = det_a_inv*(a(3,1)*a(2,3)-a(2,1)*a(3,3))
      a_inv(2,2) = det_a_inv*(a(1,1)*a(3,3)-a(3,1)*a(1,3))
      a_inv(2,3) = det_a_inv*(a(2,1)*a(1,3)-a(1,1)*a(2,3))
      a_inv(3,1) = det_a_inv*(a(2,1)*a(3,2)-a(3,1)*a(2,2))
      a_inv(3,2) = det_a_inv*(a(3,1)*a(1,2)-a(1,1)*a(3,2))
      a_inv(3,3) = det_a_inv*(a(1,1)*a(2,2)-a(2,1)*a(1,2))
C
      RETURN
      END SUBROUTINE matinv3d