Sum (sigma) notation disparity this might be a stupid question but I've googled it and I can't find an answer that I "trust". I am dealing with PCA (I guess that's not relevant, but just in case it is) and I am seeing a lot of Sigma notation that I'm not used to. My whole life I've always seen the following:
$\sum_{i=1}^{10}$
But now I am seeing this:
$\sum_{i}$
Does it mean exactly the same, supposing it is known that n=10 ?? 

When the range of the summation index is obvious/known from context, 
$$\sum_i$$ has meaning. When the summation index itself is unambiguous, even
$$\sum$$ can do.
