The Sobolev conjugate of p for , where n is space dimensionality, is
This is an important parameter in the Sobolev inequalities.
A question arises whether u from the Sobolev space belongs to for some q>p. More specifically, when does control ? It is easy to check that the following inequality
can not be true for arbitrary q. Consider , infinitely differentiable function with compact support. Introduce . We have that
The inequality (*) for results in the following inequality for
If , then by letting going to zero or infinity we obtain a contradiction. Thus the inequality (*) could only be true for
which is the Sobolev conjugate.