Fix a finite semigroup S and let a1, . . . , ak , b be tuples in a direct power S. The subpower membership problem (SMP) asks whether b can be generated by a1, . . . , ak. If S is a finite group, then there is a folklore algorithm that decides this problem in time polynomial in nk. For semigroups this problem always lies in PSPACE. We show that the SMP for a full transformation semigroup on 3 o...

We study fault tolerance properties of the group membership problem (GMP) in asynchronous systems. This problem requires each process to determine the processes with which it can communicate. We compare the properties of the GMP and consensus. We define failure sensitivity as the necessary property of a failure detector to enable a solution to the GMP. We demonstrate that the known consensus fa...

A jump system is a set of lattice points satisfying a certain exchange axiom. This notion was introduced by Bouchet and Cunningham 2], as a common generalization of (among others) the sets of bases of a matroid and degree sequences of subgraphs of a graph. We prove, under additional assumptions, a min-max formula for the distance of a lattice point from a jump system. The conditions are met in ...

The modular group plays an important role in many branches of mathematics. We show that the membership problem for the modular group is decidable in polynomial time. To this end, we develop a new syllable-based version of the known subgroup-graph approach. The new approach can be used to prove additional results. We demonstrate this by using it to prove that the membership problem for a free gr...

We prove that the Membership Problem is solvable affirmatively for every finitely generated quadratic module Q of R[X1]. For the case that the associated semialgebraic set S is bounded we show that a polynomial f is an element of Q if and only if f is nonnegative on S and fulfills certain order conditions in the boundary points of S. This leads us to the definition of generalized natural genera...

For an ideal or K-subalgebra E of K[X1, . . . , Xn], consider subfields k ⊂ K, where E is generated – as ideal or K-subalgebra – by polynomials in k[X1, . . . , Xn]. It is a standard result for ideals that there is a smallest such k. We give an algorithm to find it. We also prove that there is a smallest such k for K-subalgebras. The ideal results use reduced Gröbner bases. For the subalgebra r...

To calculate the entropy of a subalgebra or of a channel with respect to a state, one has to solve an intriguing optimalization problem. The latter is also the key part in the entanglement of formation concept, in which case the subalgebra is a subfactor. I consider some general properties, valid for these definitions in finite dimensions, and apply them to a maximal commutative subalgebra of a...