SUBROUTINE DEFER (COFX,COFY,IDMN,USOL,GRHS)
C
C     THIS SUBROUTINE FIRST APPROXIMATES THE TRUN1ATION ERROR GIVEN BY
C     TRUN1(X,Y)=DLX**2*TX+DLY**2*TY WHERE
C     TX=AFUN(X)*UXXXX/12.0+BFUN(X)*UXXX/6.0 ON THE INTERIOR AND
C     AT THE BOUNDARIES IF PERIODIC(HERE UXXX,UXXXX ARE THE THIRD
C     AND FOURTH PARTIAL DERIVATIVES OF U WITH RESPECT TO X).
C     TX IS OF THE FORM AFUN(X)/3.0*(UXXXX/4.0+UXXX/DLX)
C     AT X=A OR X=B IF THE BOUNDARY CONDITION THERE IS MIXED.
C     TX=0.0 ALONG SPECIFIED BOUNDARIES.  TY HAS SYMMETRIC FORM
C     IN Y WITH X,AFUN(X),BFUN(X) REPLACED BY Y,DFUN(Y),EFUN(Y).
C     THE SECOND ORDER SOLUTION IN USOL IS USED TO APPROXIMATE
C     (VIA SECOND ORDER FINITE DIFFERENCING) THE TRUN1ATION ERROR
C     AND THE RESULT IS ADDED TO THE RIGHT HAND SIDE IN GRHS
C     AND THEN TRANSFERRED TO USOL TO BE USED AS A NEW RIGHT
C     HAND SIDE WHEN CALLING BLKTRI FOR A FOURTH ORDER SOLUTION.
C
      COMMON /SPLP/   KSWX       ,KSWY       ,K          ,L          ,
     1                AIT        ,BIT        ,CIT        ,DIT        ,
     2                MIT        ,NIT        ,IS         ,MS         ,
     3                JS         ,NS         ,DLX        ,DLY        ,
     4                TDLX3      ,TDLY3      ,DLX4       ,DLY4
      DIMENSION       GRHS(IDMN,1)           ,USOL(IDMN,1)
      EXTERNAL        COFX       ,COFY
C
C     COMPUTE TRUN1ATION ERROR APPROXIMATION OVER THE ENTIRE MESH
C
      DO  40 J=JS,NS
         YJ = CIT+FLOAT(J-1)*DLY
         CALL COFY (YJ,DJ,EJ,FJ)
         DO  30 I=IS,MS
            XI = AIT+FLOAT(I-1)*DLX
            CALL COFX (XI,AI,BI,CI)
C
C     COMPUTE PARTIAL DERIVATIVE APPROXIMATIONS AT (XI,YJ)
C
            CALL DX (USOL,IDMN,I,J,UXXX,UXXXX)
            CALL DY (USOL,IDMN,I,J,UYYY,UYYYY)
            TX = AI*UXXXX/12.0+BI*UXXX/6.0
            TY = DJ*UYYYY/12.0+EJ*UYYY/6.0
C
C     RESET FORM OF TRUN1ATION IF AT BOUNDARY WHICH IS NON-PERIODIC
C
            IF (KSWX.EQ.1 .OR. (I.GT.1 .AND. I.LT.K)) GO TO  10
            TX = AI/3.0*(UXXXX/4.0+UXXX/DLX)
   10       IF (KSWY.EQ.1 .OR. (J.GT.1 .AND. J.LT.L)) GO TO  20
            TY = DJ/3.0*(UYYYY/4.0+UYYY/DLY)
   20       GRHS(I,J) = GRHS(I,J)+DLX**2*TX+DLY**2*TY
   30    CONTINUE
   40 CONTINUE
C
C     RESET THE RIGHT HAND SIDE IN USOL
C
      DO  60 I=IS,MS
         DO  50 J=JS,NS
            USOL(I,J) = GRHS(I,J)
   50    CONTINUE
   60 CONTINUE
      RETURN
      END

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