Let be a smooth oriented bounded domain, be the Sobolev space, and be the first eigenvalue of the bi-Laplacian operator ?2. Then for any a: 0a(O), we have and the above supremum is infinity when a?(O). This strengthens Adams' inequality in dimension 4 [D. Adams, A sharp inequality of J. Moser for high order derivatives, Ann. of Math. 128 (1988) 365�398] where he proved the above inequality holds for a=0. Moreover, we prove that for sufficiently small a an extremal function for the above inequality exists. As a special case of our results, we thus show that there exists with such that This establishes the existence of an extremal function of the original Adams inequality in dimension 4
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