The stable cooperations of Morava $K$-Theory and the fiber product of automorphism groups of formal group laws

Abstract

There are many previous studies on the Hopf algebra $K(n)_*(K(n))$, the stable cooperations of $n$th Morava $K$-theory at an odd prime. Whereas the main part of $K(n)_*(K(n))$ corepresents the group-valued functor consisting of strict automorphisms of the Honda formal group law of height $n$, relations between the whole structure of $K(n)_*(K(n))$ including the exterior part and formal group laws have not been investigated well. Firstly, we constitute a functor $C(-)$ which is given by the fiber product of two natural homomorphism between subgroups of automorphisms of formal group laws, and the Hopf algebra $C_*$ corepresenting $C(-)$. Next, we construct a Hopf algebra homomorphism $\kappa^*:C_*\to K(n)_*(K(n))$ naturally. To relate $C_*$ to $K(n)_*(K(n))$, we use stable comodule algebras which are introduced by Boardman. From the algebra structure of $K(n)_*(K(n))$ which is given by W\"urgler and Yagita, we see that $\kappa^*$ is an isomorphism. Since we formulate $C_*$ by using formal group laws, the isomorphism $\kappa^*$ clarifies relationship between the Hopf algebra structure of $K(n)_*(K(n))$ including the exterior algebra part and formal group laws.