Let G be a group and N a normal subgroup of G. We study the large scale
behavior, not the exact values themselves, of the stable mixed commutator
length sclG,Nβ on the mixed commutator subgroup [G,N]; when N=G,
sclG,Nβ equals the stable commutator length sclGβ on the commutator
subgroup [G,G]. For this purpose, we regard sclG,Nβ not only as a
function from [G,N] to Rβ₯0β, but as a bi-invariant metric
function dsclG,Nβ+β from [G,N]Γ[G,N] to Rβ₯0β.
Our main focus is coarse group theoretic structures of
([G,N],dsclG,Nβ+β). Our preliminary result (the absolute version)
connects, via the Bavard duality, ([G,N],dsclG,Nβ+β) and the quotient
vector space of the space of G-invariant quasimorphisms on N over one of
such homomorphisms. In particular, we prove that the dimension of this vector
space equals the asymptotic dimension of ([G,N],dsclG,Nβ+β).
Our main result is the comparative version: we connect the coarse kernel,
formulated by Leitner and Vigolo, of the coarse homomorphism ΞΉG,Nβ:([G,N],dsclG,Nβ+β)β([G,N],dsclGβ+β); yβ¦y, and a certain
quotient vector space W(G,N) of the space of invariant quasimorphisms. Assume
that N=[G,G] and that W(G,N) is finite dimensional with dimension β.
Then we prove that the coarse kernel of ΞΉG,Nβ is isomorphic to
Zβ as a coarse group. In contrast to the absolute version, the
space W(G,N) is finite dimensional in many cases, including all (G,N) with
finitely generated G and nilpotent G/N. As an application of our result,
given a group homomorphism Ο:GβH between finitely generated
groups, we define an R-linear map `inside' the groups, which is dual
to the naturally defined R-linear map from W(H,[H,H]) to
W(G,[G,G]) induced by Ο.Comment: 69 pages, no figure. Minor revision (v2): some symbols change