Xu introduced a class of nongraded Hamiltonian Lie algebras. These Lie algebras have a Poisson bracket structure. In this paper, the isomorphism classes of these Lie algebras are determined by employing a "sandwich" method and by studying some features of these Lie algebras. It is obtained that two Hamiltonian Lie algebras are isomorphic if and only if their corresponding Poisson algebras are isomorphic. Furthermore, the derivation algebras and the second cohomology groups are determined.

We compute the low dimensional cohomologies H q(gcN,C), Hq(gcN,C) of the infinite rank general Lie conformal algebras gcN with trivial coefficients for q<3, N51 or q<2, N>2. We also prove that the cohomology of gcN with coeffi-cients in its natural module is trivial, i.e., H*(gcN,C[э]N)=0, and thus partially solve an open problem of Bakalov-Kac-Voronov

n one of our recent papers, the associative and the Lie algebras of Weyl type A[D] = AUF[D] were de ned and studied, where A is a commutative associative algebra with an identity element over a eld F of any characteristic, and F[D] is the polynomial algebra of a commutative derivation subalgebra D of A. In the present paper, a class of the above associative and Lie algebras A[D] with F being a eld of characteristic 0, D consisting of locally nite but not locally nilpotent derivations of A, are studied. The isomorphism classes and automorphism groups of these associative and Lie algebras are determined.

We determine the derivation algebras and the isomorphism classes of a family of the simple Lie algebras introduced recently by Xu [manuscripta math. 100, 489-518 (1999)]. The structure space of these algebras is given explicitly.