(also known as " ") is a mathematical and logical puzzle loosely similar to Sudoku. The objective is to fill the grid in with the digits 1 through N (where N is the number of rows or columns in the grid) such that: - Each row contains exactly one of each digit. - Each column contains exactly one of each digit. - Each bold-outlined group of cells (block) contains digits which achieve the specified result using the specified mathematical operation: addition (+), subtraction (-), multiplication (×), and division (÷). - Unlike Killer Sudoku, digits may repeat within a block.