The idea of Van Kampen diagrams in group theory was developed by Van Kampen in the 1930s but no one really used them until the 1960s when they were rediscovered independently by Lyndon and Ol'Shanskii. Today they are an important tool in geometric group theory.

The idea of Van Kampen diagrams in group theory was developed by Van Kampen in the 1930s but no one really used them until the 1960s when they were rediscovered independently by Lyndon and Ol'shanskii. Today they are an important tool in geometric group theory.