SUBROUTINE POISP2 (M,N,A,BB,C,Q,IDIMQ,B,B2,B3,W,W2,W3,D,TCOS,P)
C
C     SUBROUTINE TO SOLVE POISSON EQUATION WITH PERIODIC BOUNDARY
C     CONDITIONS.
C
      DIMENSION       A(1)       ,BB(1)      ,C(1)       ,Q(IDIMQ,1) ,
     1                B(1)       ,B2(1)      ,B3(1)      ,W(1)       ,
     2                W2(1)      ,W3(1)      ,D(1)       ,TCOS(1)    ,
     3                P(1)
      MR = M
      NR = (N+1)/2
      NRM1 = NR-1
      IF (2*NR .NE. N) GO TO 107
C
C     EVEN NUMBER OF UNKNOWNS
C
      DO 102 J=1,NRM1
         NRMJ = NR-J
         NRPJ = NR+J
         DO 101 I=1,MR
            S = Q(I,NRMJ)-Q(I,NRPJ)
            T = Q(I,NRMJ)+Q(I,NRPJ)
            Q(I,NRMJ) = S
            Q(I,NRPJ) = T
  101    CONTINUE
  102 CONTINUE
      DO 103 I=1,MR
         Q(I,NR) = 2.*Q(I,NR)
         Q(I,N) = 2.*Q(I,N)
  103 CONTINUE
      CALL POISD2 (MR,NRM1,1,A,BB,C,Q,IDIMQ,B,W,D,TCOS,P)
      IPSTOR = W(1)
      CALL POISN2 (MR,NR+1,1,1,A,BB,C,Q(1,NR),IDIMQ,B,B2,B3,W,W2,W3,D,
     1             TCOS,P)
      IPSTOR = MAX0(IPSTOR,INT(W(1)))
      DO 105 J=1,NRM1
         NRMJ = NR-J
         NRPJ = NR+J
         DO 104 I=1,MR
            S = .5*(Q(I,NRPJ)+Q(I,NRMJ))
            T = .5*(Q(I,NRPJ)-Q(I,NRMJ))
            Q(I,NRMJ) = S
            Q(I,NRPJ) = T
  104    CONTINUE
  105 CONTINUE
      DO 106 I=1,MR
         Q(I,NR) = .5*Q(I,NR)
         Q(I,N) = .5*Q(I,N)
  106 CONTINUE
      GO TO 118
  107 CONTINUE
C
C     ODD  NUMBER OF UNKNOWNS
C
      DO 109 J=1,NRM1
         NRPJ = N+1-J
         DO 108 I=1,MR
            S = Q(I,J)-Q(I,NRPJ)
            T = Q(I,J)+Q(I,NRPJ)
            Q(I,J) = S
            Q(I,NRPJ) = T
  108    CONTINUE
  109 CONTINUE
      DO 110 I=1,MR
         Q(I,NR) = 2.*Q(I,NR)
  110 CONTINUE
      LH = NRM1/2
      DO 112 J=1,LH
         NRMJ = NR-J
         DO 111 I=1,MR
            S = Q(I,J)
            Q(I,J) = Q(I,NRMJ)
            Q(I,NRMJ) = S
  111    CONTINUE
  112 CONTINUE
      CALL POISD2 (MR,NRM1,2,A,BB,C,Q,IDIMQ,B,W,D,TCOS,P)
      IPSTOR = W(1)
      CALL POISN2 (MR,NR,2,1,A,BB,C,Q(1,NR),IDIMQ,B,B2,B3,W,W2,W3,D,
     1             TCOS,P)
      IPSTOR = MAX0(IPSTOR,INT(W(1)))
      DO 114 J=1,NRM1
         NRPJ = NR+J
         DO 113 I=1,MR
            S = .5*(Q(I,NRPJ)+Q(I,J))
            T = .5*(Q(I,NRPJ)-Q(I,J))
            Q(I,NRPJ) = T
            Q(I,J) = S
  113    CONTINUE
  114 CONTINUE
      DO 115 I=1,MR
         Q(I,NR) = .5*Q(I,NR)
  115 CONTINUE
      DO 117 J=1,LH
         NRMJ = NR-J
         DO 116 I=1,MR
            S = Q(I,J)
            Q(I,J) = Q(I,NRMJ)
            Q(I,NRMJ) = S
  116    CONTINUE
  117 CONTINUE
  118 CONTINUE
C
C     RETURN STORAGE REQUIREMENTS FOR P VECTORS.
C
      W(1) = IPSTOR
      RETURN
      END

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