# Notation for the vector space of (real) classical solutions

I am aware that this might not be the best place to ask, but I can't say I know of any other better alternative so I apologize in advance. I'm following Wald's book on QFT in curved space-time and I can't decipher the letter he's using to denote the vector space of classical solutions to the Klein-Gordon equation in curved space-time (p.57), shown in the picture below. Any ideas?

• I think it's a script S. – mike stone Apr 15 '19 at 18:24
• I honestly wonder why authors use obscure symbols for things that could probably just as easily and effectively be written using e.g. $S$. When you read something you need to be able to "hear it" in your mind and if you don't know a symbol then this just gets in the way. Oh well, I'm just ranting into the void (as usual :-) ). – StephenG Apr 15 '19 at 19:01
• I think the space of solutions or operators often use script capital letters like $\mathcal{S}$ for space of solutions, and $\mathcal{A}$ for the the algebra of operators. There are variations but that's about it. Also, as unfortunate as it is, the book did not use LaTeX typesetting we are used to these days. – Everiana Apr 15 '19 at 19:19
• Thank you, all of you. – Kandrax Apr 15 '19 at 20:31