1. Bonferroni principle
 - a shop has t different products
 - on each of their shopping, customers buy exactly v products
 - there are m customers in total, who go for shopping n many times during a year
 
Supposing customers behave randomly, what is the expected number of a pair of customers buying the exact same products duing their shopping?

2/a. What are the eigenvalues/eigenvectors of the matrix M=[1 2; 4 3]?
2/b. Is this a positive-semidefinite matrix?

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Concepts worth to know
Vectors, matrices, norms
Rank, linear independence
Eigenvalue, eigenvector, characteristic polynom
Positive semidefiniteness