JCO Website (Puzzles for the manual solver)

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This is a place chosen to make available some ideas (on puzzles, patterns, etc)

that seemed unsuitable for posting in a Forum, but some people may find interesting.

For difficult puzzles, possible solutions will not be shown (only hints).

For other puzzles, the solution wil be hidden so that a serious attempt can be made

without direct access to a solution.

(hopefully this will make the puzzles more interesting for newcomers)

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(useful to look up for the meaning of some unfamiliar concept)

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Recent Puzzles (1)

Puzzle SX66#16 (overall #141) (6x6 Sudoku-X (SE = 7.1); proposed Oct 21, 2026)

I propose the following Sudoku-X puzzle to be solved manually.

(generated and displayed with Sudoku6Explainer)

Sudoku-X

Hint (For now, just the givens)

+-------+-------+
| . . . | . . . |
| . . . | 5 . . |
+-------+-------+
| . . . | . 1 2 |
| 1 6 . | . . . |
+-------+-------+
| . . 4 | . . . |
| . . . | . . . |
+-------+-------+ Sudoku-X

Recent Problems (1)

Problem #8: Chess (Proposed Oct 11, 2026)

Consider the chess position above. Black played 10... Qxa1.

What should be white's response ? How the game should continue with correct play ?

Hint (Final update)

Initial position (and last move played):

White: Ke1, Bc1, Bf1, Rh1, Ng6, Qh5, a2, b2, c4, d2, e3, g2, h2.
Black: Ke8, Qa1, Ra8, Rh8, Nb8, Ng8, Bc8, Bf8, a7, b7, c7, f6, h7.

(Black's last move: 10... Qxa1)

Update 1 (Oct 14, 2026)

After 11.Nxh8+ Kd8!

[after 11... Kd7? 12.Be2! Nh6 13.0-0 and black seems to be in big trouble]

12.Nf7+ Ke7 13.Qxh7!

Interesting position! How to continue ?

Update 2 (Oct 17, 2026)

I considered two possibilities for black:

(A) 13... Ke8

(B) 13... Bf5

Both options lead to complex position with many ways
to continue for both sides.

. As a sample, in option (A):

14.Be2! Qxc1+ 15.Kf2 Qxh1 16.Bh5 looks very dangerous!

How should black continue in this variation ?

Update 3 (Oct 18, 2026)

Other interesting variations for analysis

for this (open ended problem) are

. In option (A): 14.Qxg8 Qxc1+ 15.Kf2 Nc6
  and now, how should white continue ?

. In option (B): 14.Qxf5 Qxc1 + 15.Kf2 Kxf7
  and now, how should white continue ?

Past Puzzles (140)

Past proposed 9x9 CLASSIC SUDOKU PUZZLES (55) are HERE
Past proposed SLITHERLINK PUZZLES (31) are HERE
Past proposed 9x9 JIGSAW SUDOKU PUZZLES (6) are HERE
Past proposed 6x6 SUDOKU-X PUZZLES (16) are HERE
Past proposed 9x9 SUDOKU-X PUZZLES (2) are HERE
Past proposed LATIN SQUARES PUZZLES (7) are HERE
Past proposed HIDOKU PUZZLES (15) are HERE
Past proposed 6x6 CLASSIC SUDOKU PUZZLES (7) are HERE
Past proposed HASHI PUZZLES (1) are HERE

Past Problems (7)

Interesting Forums ( in particular, very fond of the NSPF! )

Sudoku (The New Sudoku Player's Forum - NSPF)
Puzzles
Solving without Pencilmarks by RW
How to find ALSs to build a chain by JCO
Hard 5x5 Latin Squares by qiuyanzhe
Hardest 6x6 ? by 999_Springs
hardest 6x6 sudoku-x ? by 1to9only
AIC by Myth Jellies
Sudoku Online (AU)
Puzzling (StackExchange)

Created: February 12, 2024

Contact: sudo.jco.br@gmail.com