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3. Two-dimensional Lattices (5)
4. Two-dimensional Space Groups (17)
a) Point groups by translation on lattice (13)
b) Reflection plus translation of point group (4)
c) Symbols and Notation
∎rules Each lattice site should accept 2-D motif or point group,<br />
to form a smallest unit cell with highest symmetry elements<br />
filling the space completely just by translation. <br />
∎results 4 primitive cells (P. R. E. S)<br />
1 centered doubly primitive cell (C)<br />
① 1-fold symmetry (1) : no restriction on the lattice point <br />
Asymmetrical motif R can be repeated in an oblique arrangement.