Linked from
The 22 pages that link to Invariant (mathematics), each with the reason it gives.
IsomorphismRelated: Isomorphism invariants can distinguish structures that are not equivalent.
Mathematical puzzleRelated: Tracking an invariant can rule out impossible puzzle outcomes.
Tower of HanoiRelated: The size order on each peg must remain valid after every legal move.
Peg solitaireRelated: Peg-solitaire invariants restrict which positions a move sequence can reach.