Principle of Inclusion & Exclusion (PIE)

Source : Link1 Link2

Derangement : Link1 Link2

Mobius Function : Link1 Link2 Link3

Code : (Mobius Function)

Mobius Inversion : Link1 Link2 Link3

Problem Links : 

1. LCMSUM - LCM Sum

2. GCDEX2 - GCD Extreme (hard)

3. Hyperrectangle GCD

4. EXGCD

5. GCD Product

6. LCM

7. LCMs

8. C. Mike and Foam

9. COPRIME3

10. RANDCHCL