What pattern does this problem use?

A number transformation game repeatedly replaces a number with the sum of the squares of its digits. A number is 'stable' if this process eventually lands on 1. It's 'unstable' if it loops forever. Given a starting number, return true if it's stable. Example: 19 → (1² + 9² = 82) → (8² + 2² = 68) → ... → 1. Stable → true.