
# evaluate the spectral likelihood function using 
observations from 1 to [n/2] in the periodogram instead of 
the index from 1 to n.

# compute spectral generating function for the model 'trend+ar2'.

# document transformation of AR coefficientes in model 'trend+ar2', 
it is based on arima().

# compute gradient and Hessian for parameterization 'tpexp10sq'
and Hessian of transformation of AR coefficientes.
