In topology, a branch of mathematics, an excisive triad is a triple ( X ; A , B ) {\displaystyle (X;A,B)} of topological spaces such that A, B are subspaces of X and X is the union of the interior of A and the interior of B. Note B is not required to be a subspace of A.
See also Homotopy excision theorem
Notes
References May, J. Peter (2019). A concise course in algebraic topology. ISBN 978-7-5192-6659-2. OCLC 1200780491. Munkres, James; Topology, Prentice Hall; 2nd edition (December 28, 1999). ISBN 0-13-181629-2.