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In this paper we continue our study of hopficity begun in [1], [2], [3], [4] and [5]. Let$A$be hopfian and let$B$have a cyclic center of prime power order. We improve Theorem 4 of [2] by showing that if$B$has finitely many normal subgroups which form a chain (we say$B$is$n$-normal), then$AxB$is hopfian. We then consider the case when$B$is a$p$-group of nilpotency class 2 and show that in certain cases$AxB$is hopfian.

We prove new inequalities of the Lieb-Thirring type on the eigenvalues of Schrdinger operators in wave guides with local perturbations. The estimates are optimal in the weak-coupling case. To illustrate their applications, we consider, in particular, a straight strip and a straight circular tube with either mixed boundary conditions or boundary deformations.

We propose and analyze a multivariate encryption scheme that uses odd characteristic and an embedding in its construction. This system has a very simple core map F(X) = X^2, allowing for efficient decryption. We also discuss ways to make this decryption faster with specific parameter choices. We give heuristic arguments along with experimental data to show that this scheme resists all known attacks.