Delta-matroid
In mathematics, a delta-matroid or Δ-matroid is a family of sets obeying an exchange axiom generalizing an axiom of matroids. A non-empty family of sets is a delta-matroid if, for every two sets E {\displaystyle E} and F {\displaystyle F} in the family, and for every element e {\displaystyle e} in their symmetric difference E △ F {\displaystyle E\triangle F} , there exists an f ∈ E △ F {\displaystyle f\in E\triangle F} such that E △ { e , f } {\displaystyle E\triangle \{e,f\}} is in the family.