This note points out a new aspect of the well-known relationship between the subjects mentioned in the title. The following result and its generalization in totally real algebraic number fields is central to the discussion. Let
denote the Legendre symbol for relatively prime numbers a and b ℇ ℤ and
a substitution of the modular subgroup Γ0(4). Then, if γ>0 and b≡1 mod 2,

with

and

According to (1), the Legendre symbol behaves somewhat like a modular function ﹙apart from the known behaviour under
and
﹚. (1) follows (see below) from the functional equation

with

provided that

Here we used and always will use the abbreviation

and ℇδ means the absolutely least residue of δ mod 4. In the proof, Hecke [4] assumed γ>0 (see also Shimura [5]).