Matrices
When 2 × 2 real matrices m are used for a source, and the number one (1) is identified with the identity matrix, and minus one (−1) with the negative of the identity matrix, then there are many solutions to m2 = −1. In fact, there are many solutions to m2 = +1 and m2 = 0 also. Any such m can be taken as a basis vector, along with 1, to form a planar algebra.
Read more about this topic: Imaginary Unit