Finite-dimensionality conjecture for polynomial-growth biharmonic functions

Let $M$ be an open manifold with nonnegative Ricci curvature, and consider the vector space of biharmonic functions on $M$, where biharmonic means satisfying

$$\Delta^2u=0.$$

For a fixed growth rate, restrict to biharmonic functions with polynomial growth of that rate. Biharmonic finite-dimensionality conjecture. This space is finite dimensional. The source presents this as an extension of Yau's conjecture from harmonic to biharmonic functions on open manifolds; the resolution status is not specified.