Difficulty in understanding this summations to analyze time complexity
Shellsort is notoriously difficult to analyze (surprising for such a simple algorithm). In Knuth's "Sorting and Searching" it gets an inordinate scrutiny, papers on its performance get published regularly.

Then again, the Busy Beaver function (At most how many ones does an $n$ state Turing machine write starting on an empty tape before halting?) is known only up to $n = 4$, the current record holder with 6 states writes $3.515×10^{18267}$ ones before halting...