$L^{1}$-Integrability of $L^{2}$-Harmonic Forms and the Hopf Conjecture

In this note, we study $L^{2}$-harmonic forms on complete simply-connected Riemannian manifolds with non-positive sectional curvature.

We first establish an a priori $L^{\infty}$-estimate for such forms via Moser iteration, under the curvature bounds $-K\leq\mathrm{sec}_{g}\leq0$.

We then prove that any $L^{2}$-harmonic form which is also $L^{1}$-integrable must vanish identically.

Consequently, on the universal cover of a closed non-positively curved manifold, the $k$-th $L^{2}$-Betti number vanishes if and only if every $L^{2}$-harmonic $k$-form is $L^{1}$-integrable.

This criterion reformulates a topological vanishing statement as an analytic integrability condition.