  Motivated by recent questions about the extension of Courant's nodal domain
theorem, we revisit a theorem published by C. Sturm in 1836, which deals with
zeros of linear combination of eigenfunctions of Sturm-Liouville problems.
Although well known in the nineteenth century, this theorem seems to have been
ignored or forgotten by some of the specialists in spectral theory since the
second half of the twentieth-century. Although not specialists in History of
Sciences, we have tried to put these theorems into the context of nineteenth
century mathematics.
