why does arimax create xreg what is singletonity in arimax what model cannot be used to fit arimax arimax model singularity what model has a singularity error what model has singularity arimax model single and could not compute qr decomposition of hessian when do i apply a parametric model? which arimax model would not have a single unit of measure and could not compute a hessian decomposition can arimax be single-dimensional can arimax model compute hessian function what is singularity of arimax model arimax model single-ordering arimax model singularity does arimax apply singularity cannot compute qr decomposition what model is used to calculate single order hessian? can arimax model use xreg what model would arimax use arimax error single and could not compute qr decomposition of hessian