As has been pointed out to me, in the modern method of using
analytical continuation on its own to get a Riemann surface a system of
cuts is not used.
But this is no good if one wants to study branches of functions. Cuts
are
boundaries of a domain for branches and if such boundaries are not
defined the domain is not defined. Then without a domain the branches
themselves are
not defined apart from just their initial values. The full properties
of
branches cannot be got from such a minimal definition.
So in order for branches to be fully defined a domain has
to be provided for them. The use of cuts to produce a simply-connected
region where the monodromy theorem applies is familiar and so
single-valuedness of each branch on the domain is obtained. A branch is
just a function from the cut plane to a region of the w-plane and is
distinguished from its fellows by its initial value. When each
branch-point is given a cut their combined images in the
w-plane separate each initial value from every other. Each is in a
range disjoint from the others. However when a node has more than one
associated branch-point, which is the case when the defining relation
is non- linear in the independent variable, it ts necessary to
harmonize the cuts so that
no redundant boundaries are created. Provide one of those branch-points
with a cut. This defines boundary-elements which meet at the node. Take
one sucb boundary-element and image it with cuts terminating in the
remaining branch-points.
Permutations on branches are intrinsic to closed circuits but
one wants to associate them with crossings of cuts. NB: A permutation
cannot generally be associated consistently with just a branch-point.
Put an arrow (orientation counts) across each cut. Now take a circuit
from and to the
initial point which crosses one cut only in the direction of its arrow.
The
permutation produced by the circuit is given to the arrow. Crossing the
opposite
way gives the inverse permutation. Cuts from branch-points associated
with the same node get the same permutation.
A general circuit crosses more than one cut. The permutation
it produces is the same as the product of the assigned permutations
taken in the order in which the cuts are crossed. This is shown by
distorting the circuit back to the initial point in between crossings,
Now translatability, to get permutations for a system of cuts
from those assumed already known for another system. A circuit crossing
one cut only in the system for determination may cross more than one
cut in the known system. But its permutation is determinable by the
previous paragraph and this permutation just has to be given to the
crossed cut.
© Jan2004 and Aug2010 revision.