Published online by Cambridge University Press: 01 October 1999
Denoteby $(R,\cdot)$ the multiplicative semigroup of an associative algebra$R$ over an infinite field, and let $(R,\circ)$ represent $R$ whenviewed as a semigroup via the circle operation $x\circy=x+y+xy$. In thispaper we characterize the existence of an identity in these semigroupsin terms of the Lie structure of $R$. Namely, we prove that thefollowing conditions on $R$ are equivalent: the semigroup $(R,\circ)$satisfies an identity; the semigroup $(R,\cdot)$ satisfies a reducedidentity; and, the associated Lie algebra of $R$ satisfies the Engelcondition. When $R$ is finitely generated these conditions are eachequivalent to $R$ being upper Lienilpotent.
1991 Mathematics Subject Classification 16R40, 20M07, 20M25