Isomorphism and higher order equivalence
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M. Ajtai
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Article
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1979
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Elsevier Science
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## A (K, A) tree is normal if Vx ~ T3y, z ~ T(y fi z A y < x A z < x A h,r(y) = h,r(Z) --hT(x)+ 1), The elements p, q of an arbitrary partially ordered set P are called incompatible if they have no common lower bound in P. An antichain is a set of pairwise incompatible elements in P. B is an a-b