文摘
Let K be a field of characteristic zero, let σ be an automorphism of K and let δ be a σ-derivation of K . We show that the division ring 16001071&_mathId=si1.gif&_user=111111111&_pii=S0021869316001071&_rdoc=1&_issn=00218693&md5=c4dacbbbb78c2bcdbb71a1638557571d" title="Click to view the MathML source">D=K(x;σ,δ) either has the property that every finitely generated subring satisfies a polynomial identity or D contains a free algebra on two generators over its center. In the case when K is finitely generated over a subfield k we then see that for σ a k-algebra automorphism of K and δ a k-linear derivation of K , 16001071&_mathId=si2.gif&_user=111111111&_pii=S0021869316001071&_rdoc=1&_issn=00218693&md5=7d2a1cb19639c78e8032212ee9a31875" title="Click to view the MathML source">K(x;σ) having a free subalgebra on two generators is equivalent to σ having infinite order, and 16001071&_mathId=si24.gif&_user=111111111&_pii=S0021869316001071&_rdoc=1&_issn=00218693&md5=4066d81f6e7e24aa70d8dbd7c9a4d324" title="Click to view the MathML source">K(x;δ) having a free subalgebra is equivalent to δ being nonzero. As an application, we show that if D is a division ring with center k of characteristic zero and 16001071&_mathId=si4.gif&_user=111111111&_pii=S0021869316001071&_rdoc=1&_issn=00218693&md5=ef9351e84c96a2f5ac2dbc8c8eaed86d" title="Click to view the MathML source">D⁎ contains a solvable subgroup that is not locally abelian-by-finite, then D contains a free k-algebra on two generators. Moreover, if we assume that k is uncountable, without any restrictions on the characteristic of k, then D contains the k-group algebra of the free group of rank two.