After some confusion on my part, I wanted to know is there a profound mathematical reason why both Hardy spaces and Sobolev spaces are denoted by $H^p$(1). Is it just coincidence? Does it have any historical meaning?
Note: Granted that a Sobolev space of $k$ derivative all in $L^p$ will not be denoted this way, but usually when $p=2$ then it is omitted, and we are left with $H^k$.