A Littlewood-Paley type theorem on orthoprojectors onto mutually   orthogonal subspaces of piecewise polynomial functions and its corollary
  The article proves an assertion analogous to the Littlewood-Paley theorem for the orthoprojectors onto mutually orthogonal subspaces of piecewise polynomial functions on the cube $ I^d. $ This assertion provides an upper estimate for the norms of the functions in $ L_p(I^d) $ via corresponding norms of projections onto subspaces of piecewise polynomial multivariable functions. These relationships are used to obtain upper estimates of the Kolmogorov widths of Besov classes of non-periodic functions meeting the mixed Hoelder conditions. 
S. N. Kudryavtsev
math.CA