So, I've found on Stackexchange that: $$\mathbb{E} \left[ \mid S_n \mid \right]=\sqrt{\frac{2n}{d}}\frac{\Gamma (\frac{d+1}{2})}{\Gamma (\frac{d}{2})},$$ but I'm not sure if I can cite it in my thesis, like straight from someone's comment...
Do you know any book that provides this formula (even without proof)?
\midis for use as a binary operator, e.g. divisibility (p\mid n$\to p\mid n$), or similar settings where spacing is needed, e.g. conditional probabilities (P(A\mid B)$\to P(A\mid B)$). For single norm bars, just use|. (For double norm bars there is\|.) Also, you can get properly sized parentheses (and other paired delimiters) that adjust to the size of their content by preceding them with\leftand\right. More generally, here's a tutorial and reference for typesetting math on this site. – joriki Mar 28 '20 at 11:08