Abstract. The main goal of this paper is to study properties of the iterated integrals of modular forms in the upper halfplane, eventually multiplied by z s−1 , along geodesics connecting two cusps. This setting generalizes simultaneously the theory of modular symbols and that of multiple zeta values. §0. Introduction and summary where the sequence of differential forms in the iterated integral consists of consecutive subsequences of the form dz z , . . . , dz z , dz 1 − z of lengths m k , m k−1 , . . . , m 1 . Easy combinatorial considerations allow one to express in two different ways products ζ(l 1 , . . . , l j ) · ζ(m 1 , . . . , m k ) as linear combinations of multiple zeta values.If one uses for this the integral representation (0.2), one gets a sum over shuffles which enumerate the simplices of highest dimension occurring in the natural simplicial decomposition of the product of two integration simplices. In fact, the formal generating series for (regularized) iterated integrals (0.2) appeared in the famous Drinfeld paper [Dr2], essentially as the Drinfeld associator, and more relations for multiple zeta values were implicitly deduced there. The question about interdependence of (double) shuffle and associator relations does not seem to be settled at the moment of writing this: cf.[Ra3]. The problem of completeness of these systems of relations is equivalent to some difficult transcendence questions.
Multiple zeta values are interesting, because they and their generalizations appear in many different contexts involving mixed Tate motives ([DeGo], [T]), deformation quantization ([Kon]), knot invariants etc.0.2. Modular symbols and periods of modular forms. Let Γ be a congruence subgroup of the modular group acting upon the union H of the upper complex half-plane H and the set of cusps P 1 (Q).The quotient Γ \ H is the modular curve X Γ . Differentials of the first kind on X Γ lift to the cusp forms of weight 2 on H (multiplied by dz).The modular symbols {α, β} Γ ∈ H 1 (X Γ , Q), where α, β ∈ P 1 (Q), were introduced in [Ma1] as linear functionals on the space of differentials of the first kind obtained by lifting and integrating. The fact that one lands in H 1 (X Γ , Q) and not just H 1 (X Γ , R) is not obvious. It was proved in [Dr1] by refining a weaker argument given in [Ma1]. This is equivalent to the statement that difference of any two cusps in Γ has finite order in the Jacobian, or else that the mixed Hodge structure on H 1 (X Γ \ {cusps}, Q) is split (cf. [El]). One of the basic new insights of [Ma1] consisted in the realization that studying the action of Hecke operators on modular symbols one gets new arithmetic facts about periods and Fourier coefficients of cusp forms of weight two.The further generalizations of modular symbols proceeded, in particular, in the following directions.
3(a) In [Ma2] it was demonstrated that the same technique applies to the integrals of cusp forms of higher weight, eventually multiplied by polynomials in z, producing the similar information about their periods and...