Remarks on modular symbols for Maass wave forms

In this paper I introduce modular symbols for Maass wave cusp forms. They appear in the guise of finitely additive functions on the Boolean algebra generated by intervals with non–positive rational ends, with values in analytic functions (pseudo–measures in the sense of [MaMar2]). After explaining the basic issues and analogies in the extended Introduction, I construct modular symbols in the sec. 1 and the related L'evy–Mellin transforms in the sec. 2. The whole paper is an extended footnote to the Lewis–Zagier fundamental study [LZ2].