For humans specifically: en.wikipedia.org/wiki/List_of_systems_of_the_human_body
Definition:
- odd permutation: -1
- even permutation: 1
- not a permutation: 0. This happens iff two more more indices are repeated
Doing physics means calculating a number by 
Ciro Santilli 37 Updated 2025-05-21 +Created 1970-01-01
Ciro Santilli 37 Updated 2025-05-21 +Created 1970-01-01It does not matter how, if it is exact, or numerical, or a message from God: a number has to come out of the formulas in the end, and you have to compare it with the experimental data.
Many theoretical physicists seem to forget this in their lectures, see also: Section "How to teach and learn physics".
Lecture notes:
- www.robots.ox.ac.uk/~az/lectures/ia/lect2.pdf Lecture 2: 2D Fourier transforms and applications by A. Zisserman (2014)
How the 2D FFT works by Mike X Cohen (2017)
Source. Animations showing how the 2D Fourier transform looks like for simple inpuf functions. There are unlisted articles, also show them or only show them.