Let
H
be an acylindrically hyperbolic group without nontrivial finite normal subgroups. We show that any finite system
S
of equations with constants from
H
is equivalent to a single equation. We also show that the algebraic set associated with
S
is, up to conjugacy, a projection of the algebraic set associated with a single splitted equation (such an equation has the form
w(x_1,\ldots,x_n)=h
, where
w\in F(X)
,
h\in H
).
From this we deduce the following statement:
Let
G
be an arbitrary overgroup of the above group
H
. Then
H
is verbally closed in
G
if and only if it is algebraically closed in
G
.
These statements have interesting implications; here we give only two of them: If
H
is a non-cyclic torsion-free hyperbolic group, then every (possibly infinite) system of equations with finitely many variables and with constants from
H
is equivalent to a single equation. We give a positive solution to Problem 5.2 from the paper [J. Group Theory 17 (2014), 29–40] of Myasnikov and Roman’kov:
Verbally closed subgroups of torsion-free hyperbolic groups are retracts.
Moreover, we describe solutions of the equation
x^ny^m=a^nb^m
in acylindrically hyperbolic groups (AH-groups), where
a
,
b
are non-commensurable jointly special loxodromic elements and
n,m
are integers with sufficiently large common divisor. We also prove the existence of special test words in AH-groups and give an application to endomorphisms of AH-groups.