Dimension of divergence set of the wave equation

We consider the Hausdorff dimension of the divergence set on which the pointwise convergence lim_t→0e^it−Δf(x)=f(x) fails when f∈H^s(R^d). We especially prove the conjecture raised by Barceló et al. (2011) for d=3, and improve the previous results in higher dimensions d≥4. We also show that a Strichartz type estimate for f→e^it−Δf with the measure dtdμ(x) is essentially equivalent to the estimate for the spherical average of μ̂ which has been extensively studied for the Falconer distance set problem. The equivalence provides shortcuts to the recent results due to Liu (2019) and Rogers (2018).