sqdancefan

The double square brackets have no specific meaning except in some context. They may be used for any of ...

• nearest-integer function. [[3.4]] = 3; [[3.5]]= 4, for example
• ceiling function. [[3.4]] = 4
• floor function. [[3.6]] = 3
• identification of matrix rows and columns. [[1, 2, 3][4, 5, 6]] is a matrix of two 3-column rows
• norm function. [[3+4i]] = √(3²+4²) = 5

Check your math text for the definition it intends.