Consistency relation for $R^p$ inflation
  We consider $R^p$ inflation with $p \approx 2$, allowing small deviation from
$R^2$ inflation. Using the inflaton potential in the Einstein frame, we
construct a consistency relation between the scalar spectral index, the
tensor-to-scalar ratio, as well as the running of the scalar spectral index,
which will be useful to constrain a deviation from $R^2$ inflation in future
observations.
