The possible symmetries of arrays extending in one direction with a fixed
repeating distance.
Linear polymer chains in the crystalline state must belong to one of the line repetition groups.
Permitted symmetry elements are: the identity operation (symbol l); the translation
along the chain axis (symbol t); the mirror plane orthogonal to the chain axis (symbol

) and that containing the chain axis (symbol

); the glide plane containing the chain axis (symbol

); the
inversion centre, placed on the chain axis (symbol

); the two-fold axis orthogonal to the chain axis (symbol 2); the helical, or screw,
symmetry where the axis of the
helix coincides with the chain axis. In the latter case, the symbol is

,
where

stands for the screw axis,

is the class of the
helix, * and / are separators, and

is the integral number of residues contained in

turns, corresponding to the identity period
(

and

must be prime to each other). The class index

may be dropped if deemed unnecessary, so that the
helix may also be simply denoted as

.
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