- Unbalance cycle: a cycle with an odd number of negative edge, also referred to as a negative cycle. "unbalanced" because the social rules of friend of friend breaks down.
- Balanced set is a any set of vertices that does not induced an unbalanced cycle. A balanced cycle is also referred to as positive cycle.
- Switching at a vertex \( v \) is the operation multiplying all edges incident with \( v \) by - sign. Note: if \( e \) is a loop at \( v \), then it would be multiplied by - twice.
- Sign of a substructure is the product of the signs of its edges consider mutiplicity.
