(1)  f  = arg  min  f    ∑  i = 1  l    | | f   i   - y   i   | |  2  2   + ∑  i = 1  n    | | f   i   - ∑  j ∈ N ( i  )     W  i j   f   j   | |  2  2     s . t . ∑  j = 1  n    W  i j   = 1