{"id":"endpoint-preservation-invariant","text":"The missing-number-in-AP algorithm's correctness depends on the guarantee that the removed element is never the first or last element, so `arr[0]` and `arr[-1]` are the true endpoints of the original progression.","truth_value":"IN","source":"entries/2026/06/06/missing-number-in-arithmetic-progression-solution.md","source_url":"","source_hash":"","justifications":[],"dependents":[],"metadata":{},"created_at":"","updated_at":"","reviewed_at":"","verified_at":"","retracted_at":"","explanation":{"steps":[{"node":"endpoint-preservation-invariant","truth_value":"IN","reason":"premise"}]}}