By Vladimir Shpilrain

the most goal of this booklet is to teach how principles from combinatorial staff idea have unfold to 2 different parts of arithmetic: the speculation of Lie algebras and affine algebraic geometry. a few of these rules, in flip, got here to combinatorial staff idea from low-dimensional topology at the beginning of the 20 th Century.

This ebook is split into 3 relatively self reliant elements. half I presents a short exposition of a number of classical suggestions in combinatorial crew idea, specifically, equipment of Nielsen, Whitehead, and Tietze. half II includes the focus of the booklet. the following the authors express how the aforementioned concepts of combinatorial team idea discovered their method into affine algebraic geometry, a desirable sector of arithmetic that reports polynomials and polynomial mappings. half III illustrates how rules from combinatorial workforce concept contributed to the idea of unfastened algebras. the point of interest this is on Schreier kinds of algebras (a number of algebras is related to be Schreier if any subalgebra of a loose algebra of this kind is unfastened within the related number of algebras).

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**Sample text**

Let T induce some automorphism 'ljJ of the group F IR'. Then 'ljJ is tarne (Bachmuth et al. [29]) and hence a composition of inner automorphisms of F IR' together with the automorphisms of F IR' induced by the maps x -t y, Y -t X and x -t xy, Y -t y. It suffices, therefore, to verify that the congruence [T(X),T(Y)] == [x,y]±g mod [R',F] holds for some g in F when T is assumed to be an inner automorphism of F I[R', F] or else defined by the maps x -t y, Y -t X and x -t xy, Y -t y. These verifications are straightforward, and we omit the details.

We may clearly assume that [u, v] == [x, y] mod [R', F]. Since [R', F]- 1 :::: ßFßRßF (see, for instance, [146, p. 3) ßFßRßF). F, 8'u/8'y(v - [u, v]) + 8'v/8'y(1 - v-luv) == (1 - y-lxy) mod ßRßF. 4) yields four congruences modulo ßR given by the following matrix equation: ( 8'U/8'X 8'u/8'y 8'V/8'X) (8(V - [u, v])/8x 8'v/8'y 8(1 - v- l uv)/8x 8(v - [u, v])/8y ) 8(1 - v- l uv)/8y = (X-ly-l(y - 1) 1 - x-ly-l(X - 1)) . _y-l y-l _ y-lx It is easily verified that the matrix on the right-hand side above is invertible over ZF and hence also over ZF (mod ßR).

Suppose first that g. c. d. (aXl (u), aX2 (u)) = 1. Then, upon taking Xl and X2 to appropriate powers of Xl, we can take u to Xl by an endomorphism. Then apply another endomorphism, taking Xl to u and X2 to 1. The composition of these two endomorphisms is a retraction that takes F 2 onto the subgroup generated by u. 6. If g. c. d. (a Xl (u), a X2 (u)) > 1, then any automorphic image of u has the same property. Suppose, by way of contradiction, that u belongs to a proper retract of F 2 . Since every proper retract of F 2 is cyclic and u is not a proper power, u must be a generator of a proper retract.