sentences:  The discrepancy between the observed pairwise distances formula_1
  and the distances formula_2 over a phylogenetic tree (i.e.the sum
  of the branch lengths in the path from leaf formula_3 to leaf
  formula_4) is measured by
  where the weights formula_6 depend on the least squares method used.Least squares
  distance tree construction aims to find the tree (topology and branch lengths)
  with minimal S. This is a non-trivial problem.