We extend the discrete-time Chandrasekhar recursions for least-squares
estimation in constant parameter state-space models to a class of str
uctured time-variant state-space models, special cases of which often
arise in adaptive filtering. It can be shown that the much studied exp
onentially weighted recursive least-squares filtering problem can be r
eformulated as an estimation problem for a state-space model having th
is special time-variant structure. Other applications arise in the mul
tichannel and multidimensional adaptive filtering context.