AUTHOR: Mara D. Neusel
TITLE:  Unstable Cohen--Macaulay Algebras

	  Mathematical Research Letters
	       -- to appear --
        
We characterize Cohen--Macaulay algebras in the category $K_{fg}$ 
of unstable Noetherian algebras over the Steenrod algebra via the depth of the 
$P^*$-invariant ideals. This allows us to transfer the
Cohen--Macaulay property to suitable subalgebras. We apply this to
rings of invariants of finite groups and to the
$P^*$-inseparable closure. 

AMS CODE:  55S10 Steenrod Algebra, 13XX Commutative Rings and Algebras, 55XX Algebraic Topology}
KEY WORDS: Steenrod Algebra, Cohen--Macaulay Algebras, Unstable
           Algebras, $P^*$-Invariant Prime Ideal Spectrum,
           $P^*$-Inseparable Closure, Polynomial Invariants of Finite Groups