angusj wrote:ps: One the topic of hard to spot swordfish, the following puzzle was a killer (ie I gave up and took the hint eventually):

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`..4|...|..9`

.2.|5..|...

..6|.91|.4.

-----------

...|..2|.6.

64.|.1.|.87

.9.|8..|...

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.3.|64.|7..

...|..7|.5.

1..|...|8..

Have solved it with half a swordfish and guess work.

Now I need to know why?

Working through each number in turn:

There is a swordfish in the 1's but of no use.

There is an x-wing in the 9's but of no use.

The 4's are complete.

Using forced chains in the 2's,6's,7's and 8's leads to no definite placement.

The 5's have two rows of two candidates, not overlapping, nothing obvious.

The 3's seem to be a better approach. There are two columns of two candidates, overlapping in row nine, giving half a swordfish.

I was then able to eliminate some 3's in row nine.

Knowing that there is a swordfish, I needed a connection overlapping the other two columns.

There are two possible columns:

(r1c6)(r5c6) and (r1c7)(r5c7), there are other 3's in these columns but ignore them for the moment.

This is where I take a guess, looking at the two possible columns (r2c6)(r5c6) and (r1c7)(r5c7) and looking at what happens by eliminating the 3's from each column in turn, and by choosing the column that produces the most eliminations after the elimination of the 3's in that column leads to the solution of the puzzle. Lucky guess on my part.

Only by knowing that there was a swordfish led me along this path.

If I'd not known of the swordfish,I would be stuck.

The above is a bit messy but how to complete the other half of the swordfish in the 3's?

Any answers?