7.1 Example: A reduction by two commuting operators, for professionals.
Character analysis   (Harter Problem 1.2.6, redone).

7.2 Example: A reduction by two commuting operators, for professionals.
Projection operator analysis (Harter Problem 1.2.6, redone).

7.3 Example: the projection operator reduction of \Dn{3}

Harter's Sect.  3.3 Second stage of non-Abelian symmetry analysis
projection operators (3.2.15);
1-dimensional irreps  (3.3.6);
2-dimensional irrep  (3.3.7);
Lagrange irreps dimensionality relation (3.3.17)