KenKen

KenKen™ is a mathematical and logical puzzle loosely similar to sudoku. It was invented by a Japanese mathematics teacher Tetsuya Miyamoto as an 'instruction-free' method of training the brain and was later published in The Times in March 2008.

The objective is to fill a grid in with the digits 1 through 6 such that:

Each row contains exactly one of each digit
Each column contains exactly one of each digit
Each bold-outlined group of cells contains digits which achieve the specified result using the specified mathematical operation: addition (+), subtraction (-), multiplication (x), and division (÷). Unlike Killer sudoku, digits may repeat within a group.

If you want to have a go you can find online puzzles at Kenken.com

Kenken Books

	The "Times": KenKen: The New Brain-training Puzzle Phenomenon Puzzle books by KenKen creator Tetsuya Miyamoto. Four books of puzzles available. From Amazon.co.uk
	Will Shortz Presents the Little Gift Book of Kenken 250 logic puzzles that make you smarter from the godfather of Sudoku. From Amazon.co.uk and Amazon.com
	Will Shortz Presents Kenken 100 logic puzzles that make you smarter. From Amazon.co.uk and Amazon.com
	The "Times": Junior KenKen: Brain training for young minds by KenKen creator Tetsuya Miyamoto. From Amazon.co.uk
	Will Shortz Presents I Can KenKen! KenKen helps children improve concentration, logic and arithmetic skills. Amazon.co.uk and Amazon.com