`scalar_covariance_i_j` computes the approximate covariance at a point of the functional data for lag windows defined by i,j; a scalarized version of covariance_i_j that takes point estimates.
scalar_covariance_i_j(f_data, i, j, times)
A numeric value; the covariance of the functional data at a point for lag windows defined by i,j.
the functional data matrix with observed functions in the columns
the indices i,j in 1:T that we are computing the covariance for
A vector with 4 columns containing indices specifying which subset of f_data to consider