LOGICAL FUNCTION LINOP(NP,BETA)
C  This function determines whether an autoregressive lag operator is inver-
C  tible, by seeing if the partial autocorrelation coefficients of the
C  corresponding autoregression are all less than one in absolute value.
C  The lag operator is 1 - Beta(1)*z - Beta(2)*z^2 - ... - Beta(p)*z^p.
C  Inputs:
C  NP     Order of autoregression p
C  BETA   AR coefficients, BETA(j)=Beta(j)
C  Output:
C  LINOP  .TRUE. if invertible and .FALSE. if not
      IMPLICIT REAL*8 (A-H,O-Z)
      DIMENSION BETA(NP)
      PARAMETER(LD=100)
      COMMON/SCRA/V1(LD),V2(LD),A1(LD,LD),AD(LD,LD)
      LINOP=.FALSE.
      CALL DCOPY(NP,BETA,1,V2,1)
      IF(NP.EQ.1)GO TO 30
      DO 20 K=NP,2,-1
      RR=V2(K)
      IF(DABS(RR).GE.1.0D0)RETURN
      AA=1.0D0/(1.0D0-RR**2)
      K1=K-1
      DO 10 I=1,K1
   10 V1(I)=(V2(I)+RR*V2(K-I))*AA
   20 CALL DCOPY(K1,V1,1,V2,1)
   30 IF(DABS(V2(1)).GE.1.0D0)RETURN
      LINOP=.TRUE.
      RETURN
      END