Mike Paterson, Mark N. Wegman. Linear Unification. In Conference Record of the Eighth Annual ACM Symposium on Theory of Computing, 3-5 May 1976, Hershey, Pennsylvania, USA. pages 181-186, ACM, 1976. [doi]
A unification algorithm is described which tests a set of expressions for unifiability and which requires time and space which are only linear in the size of the input.