Cantor's proof [1] of impossibility of a surjection into the powerset. Although, calling this an algorithm may offend applied mathematicians. I would call it

Cantor's proof [1] of impossibility of a surjection into the powerset. Although, calling this an algorithm may offend applied mathematicians. I would call it an algorithm since explicitly defined sets are constructed to show the contradiction. [1] https://en.wikipedia.org/wiki/Cantor's_theorem#Proof https://en.wikipedia.org/wiki/Cantor's_theorem#Proof