53,618 research outputs found
Strings in a 2-d Extremal Black Hole
String theory on 2-d charged black holes corresponding to (SL(2)xU(1)_L)/U(1)
exact asymmetric quotient CFTs are investigated. These backgrounds can be
embedded, in particular, in a two dimensional heterotic string. In the extremal
case, the quotient CFT description captures the near horizon physics, and is
equivalent to strings in AdS_2 with a gauge field. Such string vacua possess an
infinite space-time Virasoro symmetry, and hence enhancement of global
space-time Lie symmetries to affine symmetries, in agreement with the
conjectured AdS_2/CFT_1 correspondence. We argue that the entropy of these 2-d
black holes in string theory is compatible with semi-classical results, and
show that in perturbative computations part of an incoming flux is absorbed by
the black hole. Moreover, on the way we find evidence that the 2-d heterotic
string is closely related to the N=(2,1) string, and conjecture that they are
dual.Comment: 1+22 pages, harvmac, 1 eps figure; v2: refs. added, typo correcte
Simultaneous communication in noisy channels
A sender wishes to broadcast a message of length over an alphabet to
users, where each user , should be able to receive one of
possible messages. The broadcast channel has noise for each of the users
(possibly different noise for different users), who cannot distinguish between
some pairs of letters. The vector is said to be
feasible if length encoding and decoding schemes exist enabling every user
to decode his message. A rate vector is feasible if there
exists a sequence of feasible vectors such that
. We
determine the feasible rate vectors for several different scenarios and
investigate some of their properties. An interesting case discussed is when one
user can only distinguish between all the letters in a subset of the alphabet.
Tight restrictions on the feasible rate vectors for some specific noise types
for the other users are provided. The simplest non-trivial cases of two users
and alphabet of size three are fully characterized. To this end a more general
previously known result, to which we sketch an alternative proof, is used. This
problem generalizes the study of the Shannon capacity of a graph, by
considering more than a single user
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