It is proved that every space L2 (I1, ∪ I2), where I1 and I2 are finite intervals, has a Riesz basis of complex exponentials , {λk} a sequence of real numbers. A partial result for the corresponding problem for n≧3 finite intervals is also obtained.

The aim of this paper is to prove the Theorem: Let M be a complete non compact surface without boundary in the euclidean space 3. We suppose that all geodesies of M are congruent. Then M is an affine plane in 3.

Let Γ be a finite graph together with a group Gv at each vertex v. The graph productG(Γ) is obtained from the free product of all Gv by factoring out by the normal subgroup generated by for all adjacent v, w.

In this note we construct a projective resolution for G(Γ) given projective resolutions for each Gv, and obtain some applications.