Mergesort
General Information
Algorithmic problem: Sorting based on pairwise comparison
Type of algorithm: recursion
Abstract View
Invariant: After a recursive call, the input sequence of this recursive call is sorted.
Variant: For a recursive call on a subsequence [math]\displaystyle{ S' }[/math] of [math]\displaystyle{ S }[/math], let [math]\displaystyle{ S_1' }[/math] and [math]\displaystyle{ S_2' }[/math] denote the subsequences of [math]\displaystyle{ S' }[/math] with which Mergesort is called recursively from that call. Then it is [math]\displaystyle{ |S_1'| \leq \lceil|S'| /2\rceil }[/math] and [math]\displaystyle{ |S_2'| \leq \lceil|S'| /2\rceil }[/math].
Break condition: The current subsequence of the recursive call is empty or a singleton.
Induction Basis
Abstract view: Nothing to do on an empty sequence or a singleton.
Implementation: Ditto.
Proof: An empty set or singleton is trivially sorted.