Averaged Dehn Functions for Nilpotent Groups

Gromov proposed an averaged version of the Dehn function and claimed that in many cases it should be subasymptotic to the Dehn function. Using results on random walks in nilpotent groups, we confirm this claim for most nilpotent groups. In particular, if a nilpotent group satisfies the isoperimetric inequality $\delta(l)2$ then it satisfies the averaged isoperimetric inequality $\delta^{\text{avg}}(l)<C'l^{\alpha/2}$. In the case of non-abelian free nilpotent groups, the bounds we give are asymptotically sharp.Comment: 15 pages, 1 figure; many corrections/clarifications, expanded proof of main theorem, added Proposition 4. To appear in Topolog