S11

S11 is a designation used in several disciplines to identify a particular object, model, or resource. Because it is a short alphanumeric string, it has been assigned independently by different organizations, leading to a variety of unrelated meanings.

In mathematics, S11 denotes the symmetric group on 11 elements, the group of all permutations of an 11-element set. It is a finite group of order 11!, often written as S11 or Sym(11), and appears in fields ranging from abstract algebra to geometry and combinatorics.

In electrical engineering and applied physics, S11 is an S-parameter representing the input reflection coefficient of a two-port network. It measures how much of an incident signal at port 1 is reflected back when port 2 is terminated, and is commonly used to assess impedance matching, antenna performance, and network stability. S11 is typically reported as a complex quantity or in decibels.

In biology, ribosomal protein S11 is a conserved component of the small ribosomal subunit involved in translation. It is found across domains of life and is studied for its role in ribosome assembly and function, as well as for phylogenetic analyses based on ribosomal proteins.

In transportation and infrastructure, S11 appears as a designation for routes, lines, or segments in road and transit networks in various regions. It may denote a suburban or regional service, timetable segment, or street code depending on the locale.