Sudoku Puzzles 129 - Singles, Naked and Hidden Singles

Solving methods > 1- The singles

Singles can be identified by eliminating the numbers already present on the rows, columns and blocks.
The following methods permit the solution of most 'easy' puzzles.

1.1 Naked singles

A number can be assigned to a cell when it is the only possibility, given the row, the column and the block that contain the cell in question.

In E6, only 4 is possible since the other numbers are already in the groups (row, column and block) of the cell.

1,5,6,9 can be eliminated since they are allready in the row. 2,3,8 can be eliminated since they are already in the column. 6,7,8 can be eliminated since they are already in the block. Therefore 4 remains as a naked candidate.

1.2 Hidden singles
When a cell is the only one in a group that can receive a number, it must contain that number.
Looking at candidates for a group, if one number appears only in a cell, even if other numbers are candidates thru the preceding method (1.1) then this cell must contain that number.

In the 7th block (bottom left) 8 is possible only in the center cell B8. One can thus eliminate candidates 1,2,3 and assign 8 to the cell.

The 8 is already on the 7th row in F7, on the 9th in G9 and on column A in A6. Since 5 is already in C8, there remains only B8 as a possibility for 8 in that block.

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