We establish a congruence modulo four in the real Schubert calculus on the Grassmannian of m-planes in 2m-space. This congruence holds for fibers of the Wronski map and a generalization to what we call symmetric Schubert problems. This strengthens the usual congruence modulo two for numbers of real solutions to geometric problems. It also gives examples of geometric problems given by fibers of ...

We solve the matrix equation XA+AX = 0, where A ∈ C is an arbitrary given square matrix, and we compute the dimension of its solution space. This dimension coincides with the codimension of the tangent space of the congruence orbit of A. Hence, we also obtain the (real) dimension of congruence orbits in C. As an application, we determine the generic canonical structure for congruence in C and a...

We show, in an effective way, that there exists a sequence of congruence classes ak (mod mk) such that the minimal solution n = nk of the congruence φ(n) ≡ ak (mod mk) exists and satisfies log nk/ logmk → ∞ as k → ∞. Here, φ(n) is the Euler function. This answers a question raised in [3]. We also show that every congruence class containing an even integer contains infinitely many values of the ...

An important and long-standing open problem in universal algebra asks whether every finite lattice is isomorphic to the congruence lattice of a finite algebra. Until this problem is resolved, our understanding of finite algebras is incomplete, since, given an arbitrary finite algebra, we cannot say whether there are any restrictions on the shape of its congruence lattice. If we find a finite la...

Q is any quasivariety. A congruence relation 0 on a member A of Q is a Q-congruence if A/0 G Q. The set CouqA. of all Qcongruences is closed under arbitrary intersection and hence forms a complete lattice Cong A. Q. is relatively congruence-distributive if ConjA is distributive for every A e Q.. Relatively congruence-distributive quasivarieties occur naturally in the theory of abstract data typ...

The operation ‘·’, often called fusion is distributive over join. In finite residuated lattices, fusion and join determine residuation uniquely, although residuation cannot be defined equationally from other operations. The class R of residuated lattices is a variety. It is arithmetical, has CEP, and is generated by its finite members (cf. [7]). It is also congruence 1-regular, i.e., for any co...