Published online by Cambridge University Press: 20 November 2018
Suppose that S is a locally compact Hausdorff space. A one-parameter semi-group of maps in S is a family {ϕt; t ⩾ 0} of continuous functions from S into S satisfying
(i) ϕt0ϕu = ϕt+u for t, u ⩾ 0, where the circle denotes composition, and
(ii) ϕ0 = e, the identity map on S.
A semi-group {ϕt} of maps in S is said to be
(iii) of class (C0) if ϕt(x) → x as t → 0 for each x in S,
(iv) separately continuous if the function t → ϕt(x) is continuous on [0, ∞) for each x in S, and
(v) doubly continuous if the function (t, x) → (ϕt(x) is continuous on [0, ∞) x S.