The Swedish Meteorological and Hydrological Institute (SMHI) reports daily ozone levels for a number of Swedish cities. We are particularly interested in the measurements during July, which seems like...

The Swedish Meteorological and Hydrological Institute (SMHI) reports daily ozone levels for a number of Swedish cities. We are particularly interested in the measurements during July, which seems like an “ordinary” month, with mean 335 and standard deviation 16. We assume that the observations, x₁,...,x₃₁, come from a stationary Gaussian process,
  The covariance function is stated below for a few time lags,

Suppose that we want to estimate the mean value, using only three measurements during July. Which measurements should we choose? Answer the question by computing the variance of