  This survey is a slightly extended version of the lecture given by the author
at the \emph{VI International Course of Mathematical Analysis in Andaluc\'\i a}
(CIDAMA), in September 2014. Most results are contained (in a slightly less
general setting) in the earlier paper [3] (Garrig\'os, Hartzstein, Signes,
Torrea and Viviani, Pointwise convergence to initial data of heat and Laplace
equations, Trans. Amer. Math. Soc. 368 (9) (2016), 6575-6600).
