Let Q be a connected solvable
Lie group of polynomial growth. Let also E1,…,Ep be left invariant vector fields on G
that satisfy Hörmander’s condition and denote by L = −(E12+⋯+ Ep2) the
associated sub-Laplacian and by S(x,t) the ball which is centered at x ∈ Q and it is
of radius t > 0 with respect to the control distance associated to those vector fields.
The goal of this article is to prove the following Harnack inequality: there is a
constant c > 0 such that |Eiu(x)|≤ ct−1u(x), x ∈ Q, t ≥ 1,1 ≤ i ≤ p, for all u ≥ 0
such that Lu = 0 in S(x,t). This inequality is proved by adapting some ideas from
the theory of homogenization.