THE POST-BIANCHI IDENTITIES IN THE GHP FORMALISM

As is well-known, when searching for Petrov type I vacuum solutions th e imposition of tetrad conditions leads to integrability conditions on the Bianchi identities. Such post-Bianchi equations are simplest in G HP. They are exhibited explicitly for both a ''canonical'' frame and a n ''aligned'' frame. Choosing a particular gauge, however, complicates the situation considerably; many more conditions are obtained. When t his is done for the ''canonical'' case the resulting equations, when t ranslated to NP, are the Brans-Edgar equations. How all such equations can be checked is reviewed in some detail.