Systematic FTSPAH 44 and 84

This post demonstrates two more FTSPAH unlikely to frustrate anyone following the Sysudoku order of battle.

FTSPAH 44 begins with an unusually large number of givens, which generates a correspondingly large number of 3-fills. In fact, the givens offer both the box 3-fill NE[457] and the line 3-fill c7[357].

In Sysudoku traces, we mark 3-fills by naming the unit, then listing the three missing values. Line 3-fills are harder to spot, but are more often effective.  In the blog, these differences have led to the following marking conventions:

Line 3-fills are marked in the trace, and in the fill list space on the grid when the fill isn’t immediate.

Box 3-fills are marked only in the trace, ahead of line fills, and only when a fill occurs immediately.

Both types of 3-fill are marked ahead of the 1 to 9 values list.

FTSPAH 44 can be solved in the bypass by starting with the 3-fill c7[357], but the box 3-fill NE[457] is even faster. The first run includes three filled 3-fills.

In the grid at the end of the first run, the centered pencil marks are clue effects, in reserve for retrace causes.

In the grid at the end of the first run, the centered pencil marks are clue effects, in reserve for retrace causes.

The bypass trace follows below. There is no pause in which to utilize the c6 line 3-fill.

FTSPAH 84 starts with the value list, and nothing to say until value 5, then a series of head scratching, but rewarding effects.  Two unlikely hidden dublexi keep it going.  At the end of the first leg, a naked pair C36 combines with an earlier W6 for a one hdx. Then, on the second leg headed by NW8, an amazing hidden dublex. C4 on the first leg combining with NW4 on the second, deserves a spotting gold star.

Box and line marking are easy, and FTSPAH 84 goes on to deliver some advanced satisfaction.

First, on the line marked grid, a double hidden unique rectangle, Type 2b. Each 9-slink connecting floor to ceiling forces  a reversible solution rectangle when the other UR value is placed in opposite ceiling corner. For the r8 slink, that’s 3r9c8. For the r9 slink, its 3r8c8.

The Guide explains  how slinks work in the hidden UR.


In the same area, the 3r9c8 removal creates an XY-wing that pushes 84 over the edge.



The predominant 1,2,3 values hold their relationships close until the ball starts to unravel.


Next week continues the FTSPAH review with 104 and 144.



About Sudent

I'm John Welch, a retired engineering professor, father of 3 wonderful daughters and granddad to 7 fabulous grandchildren. Sudoku analysis and illustration is a great hobby and a healthy mental challenge.
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