We define a simplicial differential calculus by generalizing divided
differences from the case of curves to the case of general maps, defined on
general topological vector spaces, or even on modules over a topological ring
K. This calculus has the advantage that the number of evaluation points growths
linearly with the degree, and not exponentially as in the classical, "cubic"
approach. In particular, it is better adapted to the case of positive
characteristic, where it permits to define Weil functors corresponding to
scalar extension from K to truncated polynomial rings K[X]/(X^{k+1}).Comment: V2: minor changes, and chapter 3: new results included; to appear in
Forum Mathematicu
