2차원 평면의 이산부분군으로 2차원격자를 포함하는 군.
Glimpses of Algebra and Geometry (Undergraduate Texts in Mathematics)
Crystallography and Cohomology of Groups
The Orbifold Notation for Two-Dimensional Groups
John H. Conway and Daniel H. Huson
Structural Chemistry, Volume 13, Numbers 3-4, August 2002 , pp. 247-257(11)
The Plane Symmetry Groups: Their Recognition and Notation