E Representation in Point Group C4

Note on E representation in
C₄ character table

2 irreducible representations of point group C₄ have complex values. A two-dimensional real-valued representation E can be constructed as direct sum of the pair complex plus conjugate complex irreducible representation.
E = E_a ⊕ E_b

C₄		E	C₄	C₂	(C₄)³
	A	1	1	1	1
	B	1	-1	1	-1
E	E_a E_b	1 1	i -i	-1 -1	-i i

Obviously the E representation is reducible. Nevertheless the E representation is treated often as irreducible representation and is called real-valued or pseudo irreducible representation. One should keep in mind that general statements for character tables fail for real-valued representations. For example:

The number of irreducible representations is usually equal the number of classes. For point group C₄ this statement is true for the complex irreducible representations. The number of real-valued irreducible representations is less than the number of classes.
The sum of the squared characters of the neutral symmetry element over all irreducible representations is equal to the total number of symmetry elements
h = ∑ Χ_i(E) Χ_i(E)
This statement is true for the complex irreducible representations but not for the real-valued irreducible representations
Reduction formula: The occurence of i-th irreducible representation in an reducible representation is given by the well known formula
c_i = 1/h ∑_R n_R χ_i(R)^irred χ(R)^red
This statement is true for the complex irreducible representations. The norm of the two-dimensional real-valued irreducible representations is 2h (instead of h) meaning that the reduction formula has to be modified:
c_i(E) = 1/(2h) ∑_R n_R χ_i(R)^irred χ(R)^red = (∑_R n_R χ_i(R)^irred χ(R)^red) / (∑_R n_R χ_i(R)^irred χ_i(R)^irred)

Last update November, 13^th 2023 by A. Gelessus, Impressum, Datenschutzerklärung/DataPrivacyStatement

Note on E representation inC4 character table

h = ∑ Χi(E) Χi(E)

ci = 1/h ∑R nR χi(R)irred χ(R)red

ci(E) = 1/(2h) ∑R nR χi(R)irred χ(R)red = (∑R nR χi(R)irred χ(R)red) / (∑R nR χi(R)irred χi(R)irred)

Note on E representation in
C₄ character table

h = ∑ Χ_i(E) Χ_i(E)

c_i = 1/h ∑_R n_R χ_i(R)^irred χ(R)^red

c_i(E) = 1/(2h) ∑_R n_R χ_i(R)^irred χ(R)^red = (∑_R n_R χ_i(R)^irred χ(R)^red) / (∑_R n_R χ_i(R)^irred χ_i(R)^irred)