Editing Landau theory (section)

===Irreducible representations===
Landau expanded his theory to consider the restraints that it imposes on the symmetries before and after a transition of second order. They need to comply with a number of requirements:
*The distorted (or ordered) symmetry needs to be a subgroup of the higher one. 
*The order parameter that embodies the distortion needs to transform as a single [[irreducible representation]] (irrep) of the parent symmetry
*The  irrep should not contain a third order invariant
*If the irrep allows for more than one fourth order invariant, the resulting symmetry minimizes a linear combination of these invariants

In the latter case more than one daughter structure should be reacheable through a continuous transition. A good example of this are the structure of MnP (space group Cmca) and the low temperature structure of NbS (space group P6<sub>3</sub>mc). They are both daughters of the NiAs-structure and their distortions transform according to the same irrep of that spacegroup.<ref>{{Cite journal |last=Franzen |first=H.F. |last2=Haas |first2=C. |last3=Jellinek |first3=F. |date=1974 |title=Phase transitions between NiAs- and MnP-type phases |journal=Phys. Rev. B |volume=10 |issue=4 |pages=1248–1251 |bibcode=1974PhRvB..10.1248F |doi=10.1103/PhysRevB.10.1248}}</ref>