Spin and Pin
A subtle technical problem afflicts some uses of orthogonal matrices. Not only are the group components with determinant +1 and −1 not connected to each other, even the +1 component, SO(n), is not simply connected (except for SO(1), which is trivial). Thus it is sometimes advantageous, or even necessary, to work with a covering group of SO(n), the spin group, Spin(n). Likewise, O(n) has covering groups, the pin groups, Pin(n). For n > 2, Spin(n) is simply connected, and thus the universal covering group for SO(n). By far the most famous example of a spin group is Spin(3), which is nothing but SU(2), or the group of unit quaternions.
The Pin and Spin groups are found within Clifford algebras, which themselves can be built from orthogonal matrices.
Read more about this topic: Orthogonal Matrix
Famous quotes containing the words spin and, spin and/or pin:
“Spin and die,
To live again as butterfly.”
—Christina Georgina Rossetti (1830–1894)
“Words can have no single fixed meaning. Like wayward electrons, they can spin away from their initial orbit and enter a wider magnetic field. No one owns them or has a proprietary right to dictate how they will be used.”
—David Lehman (b. 1948)
“Suddenly we have a baby who poops and cries, and we are trying to calm, clean up, and pin things together all at once. Then as fast as we learn to cope—so soon—it is hard to recall why diapers ever seemed so important. The frontiers change, and now perhaps we have a teenager we can’t reach.”
—Polly Berrien Berends (20th century)