a(n) is the maximum number of key comparisons required to perform an indirect sort of n records with distinct keys using a two-way merge (A. D. Woodall's mergesort).
There are six places in the Algol 60 procedure mergesort where the keys are compared. The sequence shows the maxima of the counts of these comparisons, determined over all n! possible orders of the records.
a(16) >= 56.
REFERENCES
D. E. Knuth, The Art of Computer Programming Second Edition. Vol. 3, Sorting and Searching. Chapter 5.2.4 Sorting by Merging, Pages 164-166. Addison-Wesley, Reading, MA, 1998.
