class_docstr: ARMA model with exact loglikelhood for short time series
  
  Inverts (nobs, nobs) matrix, use only for nobs <= 200 or so.
  
  This class is a pattern for small sample GLS-like models. Intended use
  for loglikelihood of initial observations for ARMA.
  
  
  
  TODO:
  This might be missing the error variance. Does it assume error is
     distributed N(0,1)
  Maybe extend to mean handling, or assume it is already removed.
class_docstr_tok: ["ARMA","model","with","exact","loglikelhood","for","short","time","series","Inverts","(","nobs",",","nobs",")","matrix",",","use","only","for","nobs","<","=","200","or","so",".","This","class","is","a","pattern","for","small","sample","GLS-like","models",".","Intended","use","for","loglikelihood","of","initial","observations","for","ARMA",".","TODO",":","This","might","be","missing","the","error","variance",".","Does","it","assume","error","is","distributed","N","(","0,1",")","Maybe","extend","to","mean","handling",",","or","assume","it","is","already","removed","."]
docstr: get autocovariance matrix from ARMA regression parameter
  
  ar parameters are assumed to have rhs parameterization
docstr_tok: ["get","autocovariance","matrix","from","ARMA","regression","parameter","ar","parameters","are","assumed","to","have","rhs","parameterization"]
code: def _params2cov(self, params, nobs):
      """get autocovariance matrix from ARMA regression parameter
  
      ar parameters are assumed to have rhs parameterization
  
      """
      ar = np.r_[[1], -params[: self.nar]]
      ma = np.r_[[1], params[-self.nma :]]
      # print('ar', ar
      # print('ma', ma
      # print('nobs', nobs
      autocov = arma_acovf(ar, ma, nobs=nobs)
      # print('arma_acovf(%r, %r, nobs=%d)' % (ar, ma, nobs)
      # print(autocov.shape
      # something is strange  fixed in aram_acovf
      autocov = autocov[:nobs]
      sigma = toeplitz(autocov)
      return sigma
  
file_path: statsmodels/statsmodels/miscmodels/try_mlecov.py
imports: import numpy
  from scipy import linalg
  from scipy.linalg import toeplitz
  from statsmodels.base.model import GenericLikelihoodModel
  from statsmodels.datasets import sunspots
  from statsmodels.tsa.arima_process import ArmaProcess, arma_acovf, arma_generate_sample
label_desc: Use image node_id 1 for calling the MLEGLS obj's underlying member method code with example usage: obj._params2cov(params, nobs) and returns: sigma