x•x

x•x has two common meanings:

• If x is a real (scalar) variable, x•x = x^2.
• If x is a vector, x•x is the dot product of x with itself, which equals the squared norm: x•x = ||x||^2 = sum_i x_i^2.

Examples:

• Scalar: 3•3 = 9
• Vector: (1, 2, -3)•(1, 2, -3) = 1^2 + 2^2 + (-3)^2 = 14; ||(1,2,-3)||^2 = 14

If you meant something else (e.g., a matrix), let me know.