Inclusion-Exclusion Ladder¶
Who This Is For¶
Use this ladder when you can already do direct constructive counting, but overlapping forbidden conditions are starting to defeat simpler casework.
Warm-Up¶
- one forbidden set
- two overlapping bad sets
Core¶
- subset iteration over conditions
- count intersections cleanly
Stretch¶
- combine with bitmask DP or number theory
Example Notes¶
Exit Criteria¶
You are ready to move on when you can:
- write the sign pattern without hesitation
- define what each subset means
- compute an intersection size cleanly
- tell when inclusion-exclusion is overkill and a direct count is simpler