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## Cross Sums Sudoku

So the question is: Does giving totals for all outer 3 cells (across and/or down) in selected 9×9 squares give the ability to solve a sudoku puzzle with a reduced collection of starting numbers? Because the interior (to the left of the 4th column and above the 4th row) has no space to specify their sums, I’ve chosen normal starting numbers instead.

The book by Xin-She Yang, Ph.D., called Cryptic Kakuro and Cross Sums Sudoku offers cross sums of all the 9×9 squares by creating wider detachments between them. There are no starting numbers as a result.

This puzzle also differs from traditional Kakuro Puzzles in that the entire 9×9 grid is used. In Kakuro, only a subset of the grid is used. See my cartoon on Kakuro last May 27, 2007.

Although I tried to offer all possible sums (combinations) of 3 unique single digits 1 through 9, there are too many to list. Trust in symmetry to help furnish the rest of them, if needed.

In other news, the 3rd Annual World Championship Sudoku Competition has just provided some downloadable materials for study and sample variations that (we and) the contestants may work on. Note that in the individual competition, Sudoku variants score double the points of Classical Sudoku puzzles where the expected solving time is the same.

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## Repeat-Letter(s) Sudoku

A (Mensa!) Puzzle Book by Walter Mackay called Repeat-Letter Sudoku caught my eye this week.

This book offers puzzles where there is one (or more) doubled letter(s). In the Cartoon, there are two repeated letters: U and A. No matter. Just do the Sudoku Puzzle as usual and treat each of the pair as two different numbers.

It may help to write the same letters in unique ways: one in block letters and one in cursive or lower case. Alternatively, you can just keep count in every row, column, and block that duplicated letters show up exactly twice.

I like the effect of the no duplicates rule in the original sudoku being relaxed. It forces me to look anew at the puzzle and how it can be done, without prior built up procedural blinders. Interestingly enough, this particular variant appears studiously ignored by the rest of the internet.

I suppose if this carried far enough, it becomes simple to solve (e.g. All but one letter is duplicated 8 times). Don’t do too many of these; you may have to complain to your Eye Doctor that you’re seeing double!

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## Holographic Sudoku

This is a derivative of Twin Sudoku with only digits in each starting puzzle. A Book of such puzzles, called Unico Sudoku by Luigi Poderico is offered on the Lulu website. The introduction is written in the style of “Borat” (Amusing, not insulting).

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## Usenet And Sudoku Susser

Before the Web was in evidence in 1991 (see history), there was Usenet News, which started in 1979. It grew to consist of myriad newsgroups, even in the 1980s. Computer servers, as a public service, dedicated some of their storage to a rolling database of hundreds and later thousands of newsgroups containing messages going back several days to several weeks before the oldest messages were sloughed off in favor of the latest messages. The mechanism used was a form of uucp (Unix to Unix Copy) which stored and forwarded the message base from one Computer system to another.

Within Usenet, there existed binary newsgroups that allowed freeware, shareware, updates to existing applications that were regularly made available. See a short history of Usenet.

A while back, I was reading the newsgroup: rec.puzzles, a very large online discussion group, I searched for the subset of articles that involved Sudoku (using the Unison Newsreader on Mac OSX 10.3.9). From these (over a thousand messages), There was a message relating to the Multi-Operating System product (Freeware) called Sudoku Susser by Robert Woodhead.

He is the author of Two Sudoku books: Sudoku To Go! and Brainiac’s Sudoku Puzzle Book. The first is an adult puzzle book and the second one is more for kids or novice learners.

I’m impressed with Sudoku Susser. It is interesting because of its user interface support for various intermediate and advanced solving techniques. You can also drag and drop puzzles to solve from the Web or as files and the software will attempt to read it in.

Since the author of the books is the author of the software, Supplying the ISBN number to either book will automatically load all the puzzles in that book! The type of freeware requested is called tipware. I.e., pay what you can, if you want. I like this software’s user interface even better than that of the Seattle Times Sudoku Puzzle (which they offer via content.uclick.com), which is my favorite online web interface.

I solved the initial easy puzzle to test how the default interface worked. Every move you make is tracked. All possibilities (variable sized depending on how many) are specified in the non-starting number squares, making the puzzle a fast pleasure to solve. When highlighting a square, the non-possibilities are also shown to verify your candidate value. There are options for toggling “annoying” sound effects and an “embarrassing” timer.