A fairly basic notation question: what do the vertical bars in the following mean?
x = $_{k=0}^{3}\big|\;f(k)\;\big|$
I've never seen vertical bars with an index before and I can't seem to find the answer on google either (half the problem is that I don't know what I'm looking for!).
Encountered on page 3, equations 3-7 in this research paper. The most sensible interpretation I can think of is sum then take the absolute value.
It definitely looks like a $\sum$ is missing. You are supposed to sum up the four terms with $k$ running from $0$ to $3$, then take the absolute value. The sum indices then should be inside the absolute value signs.