Easter Monster is an “immensely difficult” Sudoku that “was created by JPF in April of 2007,” as the Sudoku Snake website explains. When it appeared, it was “hailed … as one of the world's hardest.” It was a milestone in “a new age of extremely difficult Sudokus. It was discovered that putting givens in diagonal arrangements with respect to the individual boxes would generate puzzles far harder than any yet known up to that point.” Over the years this puzzle has indeed proved very difficult to solve.

Sudoku Snake conducted “much study” on Easter Monster, and in the process, it “discovered the solving techniques Naked Double Loops and Hidden Double Loops.” The site adds, somewhat cryptically, that “with naked double loops employed, Easter Monster becomes much easier, though still extremely difficult to solve.” It is not clear to me from this whether the Sudoku Snake application is in fact able to solve the puzzle.

I have learned much from Andrew Stuart’s excellent Sudoku solver (https://www.sudokuwiki.org/Sudoku.htm). Stuart explains that one technique he has incorporated, “SK Loops,” was “discovered (invented?) trying to solve the Easter Monster.” However, while this Sudoku helped to improve the solver, the solver is nevertheless still able to fill in only one cell of Easter Monster after the givens. Stuart explains about his overall endeavors: “Since I first studied Sudoku in May 2005, I think I've finally got a handle on it. My original intention was to prove to myself that a small number of simple strategies existed that could solve every Sudoku. How wrong I was. Sudoku has enormous depth, and while this solver has grown up enough to crack 99.9% of puzzles, there are many weird and wonderful examples that defeat it.” Easter Monster remains, at least for now, among them.

I am aware of one published solution to Easter Monster. Several months after the puzzle first appeared, the Sudoku Online blog ran a series of posts chronicling an attempt to solve it. This endeavor ultimately required seven posts that totaled nearly 8,000 words and included many charts, tables, and formulas. The process was moved forward at key points by what I would call “reasonable conjectures.” For example, at one point it was necessary to “decide temporarily” that a certain situation held; at another, the solver pursued a given path because it “seemed more likely to be fruitful than others.” In the end, Sudoku Online did find the solution through this very elaborate analysis. (All the posts are indexed in the final installment, https://sudoku.com.au/Finalshotsupo...)

For my own solution, only simple techniques are required. I first analyze the givens to explore the relationships between them. As the video shows, I recognize two constraints, and this enables me to reduce the possibilities to a handful that can be tested in a relatively short time. This testing identifies the right combination of numbers that must go in certain key cells, and that in turn allows for a solution path that requires little more than crosshatching. (A short, simple forcing chain moves the solution through the only impasse it reaches.)

I will acknowledge that it helped greatly that I had seen the same general pattern found in Easter Monster in some of the other Sudokus for which there are videos on this channel. (See videos 34, 35, 36, 37, and 38.) Since Easter Monster seems to have been such a groundbreaking puzzle, it may be that it was the inspiration for these other Sudokus. However, I do not have specific information about dates of publication in order to confirm that.

I also do not know the identity of JPF. That was the user name on Sudoku forums of the creator of this puzzle. But whoever he or she was: I salute you.

Chapters:
00:00 First constraint
00:27 Second constraint
00:53 Narrowing down the possibilities
01:11 Start of solution path
02:55 Breaking through an impasse