title: Coefficient matrix not invertible with a corARMA model
body: <p>I am currently working on a project involving the <code>nlme</code> package in R. The acf plots are showing that there is autocorrelation, so I've decided to define an autocorrelation structure by using an autoregressive moving average (ARMA) correlation structure.</p>
  <p>I am trying to select the best model of the several models I am running, but I am running into issues with corARMA. Here's the chunk of ARMA correlation models I'm running:</p>
  <pre><code>mod2 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=0, q=1))
  mod3 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=1, q=0))
  mod4 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=0, q=2))
  mod5 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=2, q=0))
  mod6 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=1, q=2))
  mod7 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=2, q=1))
  mod8 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=2, q=2))
  mod9 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=0, q=3))
  mod10 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=3, q=0))
  mod11 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=1, q=3))
  mod12 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=3, q=1))
  mod13 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=2, q=3))
  mod14 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=3, q=2))
  mod15 &lt;-update(GLS_Mod1.1, corr=corARMA(form=~1|INCUB_YEAR, p=3, q=3))
  </code></pre>
  <p><code>mod7</code> is returning an error:</p>
  <pre><code>Error in `coef&lt;-.corARMA`(`*tmp*`, value = value[parMap[, i]]) : 
    Coefficient matrix not invertible
  </code></pre>
  <p><code>mod8</code> is also returning the same error.</p>
  <p>I'm not sure what this means and how I can get rid of this error. Does it have something to do with the p and q?</p>
  <p>UPDATE: I have added a <code>mod16</code> which has a p = 1 and a q = 1, yet it is returning the same <code>Coefficient matrix not invertible</code> error. Is my dataset/time series just too complicated for <code>nlme</code>?</p>