Non-linear Gradient Algorithm for Parameter Estimation: Extended version

Gradient algorithms are classical in adaptive control and parameter estimation. For instantaneous quadratic cost functions they lead to a linear time-varying dynamic system that converges exponentially under persistence of excitation conditions. In this paper we consider (instantaneous) non-quadratic cost functions, for which the gradient algorithm leads to non-linear (and non Lipschitz) time-varying dynamics, which are homogeneous in the state. We show that under persistence of excitation conditions they also converge globally, uniformly and asymptotically. Compared to the linear counterpart, they accelerate the convergence and can provide for finite-time or fixed-time stability.