Ciro Santilli @cirosantilli  40

This one is not generally seen by software, which mostly operates starting from OSI layer 2.

Complex analogue of orthogonal matrix.

Applications:

Intuitive definition: real group of rotations + reflections.

Mathematical definition that most directly represents this: the orthogonal group is the group of all matrices that preserve the dot product.

From Section "Lie algebra of a isometry group" we reach:

Group of rotations of a rigid body.

Like orthogonal group but without reflections. So it is a "special case" of the orthogonal group.

This is a subgroup of both the orthogonal group and the special linear group.

Group of the unitary matrices.

Complex analogue of the orthogonal group.

One notable difference from the orthogonal group however is that the unitary group is connected "because" its determinant is not fixed to two disconnected values 1/-1, but rather goes around in a continuous unit circle. $U(1)$ is the unit circle.

Active compound in pepper.

A more precise term for those in the know: open source software that also has a liberal license, for some definition of liberal.

Ciro Santilli defines liberal as: "can be commercialized without paying anything back" (but possibly subject to other restrictions).

He therefore does not consider Creative Commons licenses with NC to be FOSS.

For the newbs, the term open source software is good enough, since most open source software is also FOSS.

But when it's not, it's crucial to know.