This is misleading and false when it comes to Course 18. Math classes numbering has much more structure.

18.XYZ where Z >= 5 are grad classes.

    X = 1 is Analysis
    X = 2 is Mathematical Physics
    X = 3 is Applied Mathematics
    X = 4 is Theoretical Computer Science
    X = 5 is Logic
    X = 7 is Algebra
    X = 9 is Topology