Published online by Cambridge University Press: 16 October 2000
The Dickson algebra Wn+1 of invariants in a polynomial algebra over []2 is an unstable algebra over the mod 2 Steenrod algebra [Ascr], or equivalently, over the Kudo–Araki–May algebra [Kscr] of ‘lower’ operations. We prove that Wn+1 is a free unstable algebra on a certain cyclic module, modulo just one additional relation. To achieve this, we analyse the interplay of actions over [Ascr] and [Kscr] to characterize unstable cyclic modules with trivial action by the subalgebra [Ascr]n−2 on a fundamental class in degree 2n – a. This involves a new family of left ideals [Iscr]a in [Kscr], which play the role filled by the ideals [Ascr][Ascr]n−2 in the Steenrod algebra.