Being a physicist I'm still puzzled by the connection  between:

1. [Wick theorem][1] -- which is combinatorics (for me).
2. Multivariate [Gaussian integrals][2] -- which is calculus (for me).  
3. Determinants and eigensystems  -- which is linear algebra (for me).


[1]:http://en.wikipedia.org/wiki/Wick_theorem
[2]:http://en.wikipedia.org/wiki/Gaussian_integral