The review of the unspeakable FTSPAH continues with two more puzzles selected arithmetically.
The basic trace of FTSPAH 104 is Sysudoku Basic at its best. A near collapse in the bypass is extended by a deferred hidden dublex, which 104 survives, only to collapse in early line marking.
The hidden dublex leads the third run in a very productive bypass.
Imagine being at the SEhdx3 grid as a trace reader, with the task of accounting for SE3. Is your conception of the hidden dublex sufficient for spotting this one. It may need tweaking.
Now just before the point where SEhdx3 is pulled out as a cause of the SW3 list, the trace identifies a hidden single. It must be spotted in reaction to SE2. Would you have spotted it without the trace?
When you do, you’re closer to the end than you might think.
FTSPAH 144 offers an early spotting challenge, namely N3 in this grid.
Later in line marking, something harder, a hidden pair r5hp59. A hidden subset is n values confined to n cells, and therefore claiming them. More abstractly, it’s n values that can be placed in only one way.
Much easier to spot, and the equivalent, is the corresponding naked triple r5nt246.
Here is the basic trace leading to these spotting chances. The systematic and repeatable process of Sysudoku Basic, with a focused response to each grid change, replaces unfocused search in a sea of candidates.
The horde of bv on the line marked grid invite two equally systematic advanced methods, XY chains and coloring.
The XY railway delivers less than expected, but the XY ANL elimination 4r1c8 is decisive in coloring.
In the blue/green cluster the trap at r3c8 leaves a hidden single 4 in c8.
expands the cluster to wrap green in c7. The blue collapse is immediate
Next is FTSPAH 64 and 164.
Both are finished in Basic, and one in the bypass.