This is a variation of Sudoku on a "standard" 9x9 grid which contains a set of
special clue-numbers always placed in the intersections
of the grid lines. Each clue-number is the sum of all
numbers in all cells adjacent to this clue-number. So,
depending on its position in the puzzle, a clue-number
can be either the sum of four numbers in the four
adjacent squares, or the sum of two numbers in the two
adjacent squares (at the edge of the puzzle).

For example, clue-number 23 in the bottom left corner of
the puzzle means that the numbers in the four cells
adjacent to this clue-number add up to 23. Similarly,
either clue-number 9 at the top edge of the puzzle shows
the sum of the two numbers in the two cells adjacent to
this clue-number what means that possible pairs of
numbers for these cells should be chosen from the
following combinations: 1 and 8; 2 and 7; 3 and 6; 4 and
5; 5 and 4; 6 and 3; 7 and 2; or 8 and 1.

The remaining rules are as in a "standard" Sudoku, and
the object of the puzzle is to fill in the whole 9x9
grid with numbers 1 through 9 (one number per cell) so
that each horizontal line, each vertical line, and each
of the nine 3x3 squares (outlined with the bold lines)
must contain all the nine different numbers 1 through 9.

To solve the puzzle you can print the above grid.
For this click the image of the grid to go to a new
window with this grid; then you can print it.

**) Henry Kwok is the author of
this original Sudoku variant. Its name and appearance
are the original creations first published here, at my
website,
AgeOfPuzzles.com.