
Invariant functions on free groups and special HNNextensions
D. Z. Kagan^{} ^{} Moscow State University of Railway Communications
Abstract:
In
this paper we are considering questions about the possibility of
existence of invariant nontrivial pseudocharacters on free groups.
It is proved that nontrivial pseudocharacters exist on a certain
type of HNNexpansions in complex cases.
We got some results about the width of verbal subgroups generated
by words from commutator subgroup and nontriviality of the second
group of bounded cohomologies for considered HNNexpansions. Thus,
partial answer to the question, formulated R. I. Grigorchuk, is
received.
Pseudocharacter is the real functions $f$ from group $G$ to
$\mathcal{R}$ such that $f(xy)  f(x)  f(y)\leq \varepsilon$
for some $\varepsilon>0$ and for any $ x,y\in G$ and $ f(x^n) =
nf(x)$ $\forall n\in\mathbb{Z}$, $\forall x\in G$. A
pseudocharacter is called nontrivial if
$\varphi(ab)\varphi(a)\varphi(b)\neq 0$ for some $a, b\in G$.
Existence of nontrivial pseudocharacters on a group is connected
with many important characteristics and properties of groups.
The notion of pseudocharacter was introduced by A. I. Shtern.
Sufficient conditions of the existence of nontrivial
pseudocharacters for free products with amalgamation and
HNNextensions for which associated subgroups are different from
the base group were found by R. I. Grigorchuk and V.G. Bardakov.
Nontrivial pseudocharacters exist on groups with one defining
relation, and at least three generators.
Questions about conditions of existence of nontrivial
pseudocharacters for groups with one defining relation and two
generators and for descending HNNextensions remain open. These
questions in many cases are reduced to constructing nontrivial
pseudocharacters on free groups, invariant with respect to special
type of endomorphisms.
In this paper we prove existence of nontrivial pseudocharacters
for free groups $F_n$, $n>1$, which are invariant with respect to
certain types of endomorphisms. It is proved that nontrivial
pseudocharacters exist on some descending HNNextensions.
This work is devoted to the seventieth Doctor of Physical and Mathematical Sciences, Professor Vasily Ivanovich Bernik. In her curriculum vitae, a brief analysis of his scientific work and educational and organizational activities. The work included a list of 80 major scientific
works of V. I. Bernik.
Bibliography: 17 titles.
Keywords:
nontrivial pseudocharacters, free groups,bounded cohomologies, width of verbal subgroups, HNNextensions.
DOI:
https://doi.org/10.22405/222683832017181109122
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UDC:
512.543 Received: 25.06.2016 Accepted:14.03.2017
Citation:
D. Z. Kagan, “Invariant functions on free groups and special HNNextensions”, Chebyshevskii Sb., 18:1 (2017), 109–122
Citation in format AMSBIB
\Bibitem{Kag17}
\by D.~Z.~Kagan
\paper Invariant functions on free groups and special HNNextensions
\jour Chebyshevskii Sb.
\yr 2017
\vol 18
\issue 1
\pages 109122
\mathnet{http://mi.mathnet.ru/cheb536}
\crossref{https://doi.org/10.22405/222683832017181109122}
\elib{http://elibrary.ru/item.asp?id=29119839}
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http://mi.mathnet.ru/eng/cheb536 http://mi.mathnet.ru/eng/cheb/v18/i1/p109
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