[quote]Contains some puzzles that cannot be solved by logic alone, sudoku should not need trial and error[/quote]

and got response:

[quote]We assure you that every puzzle has a unique solution - one to be found with logic alone. There are NO unsolvable puzzles. If you come across a puzzle YOU deem unsolvable, feel free to post it to our forums (http://genina.com/forums/show/1.page) and we will show you how it is solved. [/quote]

so I played a few games (Extreme) until I came to this one:

Based on the "assurance" that every puzzle can be found by logic I concluded that when I got stuck it must be me at fault so went for the "hint" option.

That - for no reason that I can understand - decided that in the cells I have put the "pencil marks" in red, the 4s and 6s could be dispensed with - and that "therefore" the 4 was to be found at the very head of that column.

Can anyone explain this to me?

[img]http://i623.photobucket.com/albums/tt318/porkypete58/unplay_zps4ebd0e34.png [/img]]]>

That's why none of the cells you've marked with red pencil marks can contain 4 or 6. (This also means you can safely remove a 6 as an option for the center cell in the very last row, thus giving you another couple of cells to solve easily)

By the way, same would be true for 3 cells in the same block containing the same set of 3 numbers and so on.

Hope this helps.]]>

The puzzle as given.]]>

The '4' and '6' in r46c5 form a naked pair and eliminate all 4's and 6's in Box 5 and column 5.]]>