  This is an expository article that results from a talk given to second year
students at Oldenburg university. The aim of the talk was to show what
beautiful and unexpected results may be obtained if one plays with daring
analogies in a way that is usually not done in undergraduate education
(unfortunately): We start from the fact that the sum of diagonal entries of a
symmetric matrix equals the sum of its eigenvalues. We then guess an analogous
formula where the matrix is replaced by a function of two real variables and
sums are replaced by integrals in a systematic way. We show that this is indeed
a worthwhile process: In a special case it yields that the sum of inverse
squares of the positive integers is pi^2/6. Finally, an outline of the proof of
the guessed formula is given, and further applications, for example to the
connection between billiards and the frequencies of a drum, are explained.
