Definition
Plain language
Something about a system that is supposed to stay true no matter what happens to it.
As stated in the literature
A property that holds across all reachable states of a program or system; proving an invariant is often the crux of a correctness proof, and much of the difficulty in verified systems and olympiad combinatorics is finding the right one.
Also called: invariants
Why it matters: It matters because finding the property that always stays true is often the hardest and most crucial step in proving a system correct.
For example, a bank system should always keep the invariant that no account balance ever drops below zero.
Heard on the show
“The model is weakest when the core difficulty is preserving combinatorial structure across many cases, or proving an invariant that has to be tuned just so.”Episode 048 — How a 30B Open Model Reached Olympiad Gold With the Right Recipe