  Let $P$ be orthogonal projection on B-splines of degree $r-1$ with equally
spaced knots. Sweldens and Piessens proved that $P(x^r)-x^r$ is Bernoulli
polynomial. We generalize Sweldens ans Piessens's result for box-splines. It
gives the opportunity to define the seminorm of Sobolev space in terms of the
asymptotic formula for the error in orthogonal projection.
