  During a first St. Petersburg period Leonhard Euler, in his early twenties,
became interested in the Basel problem: summing the series of inverse squares
(posed by Pietro Mengoli in mid 17th century). In the words of Andre Weil
(1989) "as with most questions that ever attracted his attention, he never
abandoned it". Euler introduced on the way the alternating "phi-series", the
better converging companion of the zeta function, the first example of a
polylogarithm at a root of unity. He realized - empirically! - that odd zeta
values appear to be new (transcendental?) numbers. It is amazing to see how, a
quarter of a millennium later, the numbers Euler played with, "however
repugnant" this game might have seemed to his contemporary lovers of the
"higher kind of calculus", reappeared in the analytic calculation of the
anomalous magnetic moment of the electron, the most precisely calculated and
measured physical quantity. Mathematicians, inspired by ideas of Grothendieck,
are reviving the dream of Galois of uncovering a group structure in the ring of
periods (that includes the multiple zeta values) - applied to the study of
Feynman amplitudes.
