arima problems when using argument fixed=

As I am reading ?arima, only NA entries in the argument fixed= 
imports. The following seems to indicate otherwise:



 x <- arima.sim(model=list(ar=0.8), n=100) + (1:100)/50

t <- 1:100
mod1 <- lm(x ~ t)

init1  <- c(0, coef(mod1)[2])
fixed1 <- c(as.numeric(NA), 0)

arima(x, order=c(1,0,0), xreg=t, include.mean=FALSE, init=init1, 

fixed=fixed1)

Call:
arima(x = x, order = c(1, 0, 0), xreg = t, include.mean = FALSE, 
fixed = fixed1, 
    init = init1)

Coefficients:
         ar1  t
      0.9281  0
s.e.  0.0357  0

sigma^2 estimated as 0.9186:  log likelihood = -138.64,  aic = 281.28

init2 <- init1
init2[2] <- 0

fixed2 <- init1
fixed2[1] <- as.numeric(NA)

arima(x, order=c(1,0,0), xreg=t, include.mean=FALSE, init=init2, 

fixed=fixed2)

Call:
arima(x = x, order = c(1, 0, 0), xreg = t, include.mean = FALSE, 
fixed = fixed2, 
    init = init2)

Coefficients:
         ar1       t
      0.7888  0.0377
s.e.  0.0593  0.0000

sigma^2 estimated as 0.8452:  log likelihood = -133.97,  aic = 271.94

fixed1

[1] NA  0

fixed2

t 
        NA 0.03767406

arima(x, order=c(1,0,0), xreg=t, include.mean=FALSE, init=init2, 

fixed=fixed1)

Call:
arima(x = x, order = c(1, 0, 0), xreg = t, include.mean = FALSE, 
fixed = fixed1, 
    init = init2)

Coefficients:
         ar1  t
      0.9281  0
s.e.  0.0357  0

sigma^2 estimated as 0.9186:  log likelihood = -138.64,  aic = 281.28


Kjetil Halvorsen