(R,+)
GR
0∈G, ∀(x, y)∈G2:x+y∈G, ∀x∈G:−x∈G
x∈G q ∈Zng ∈G
x−x
A B RA B
∀b∈B, ∀ε > 0:]b−ε, b +ε[∩A6=∅
G(R,+) G α
G∩]0, α[= ∅
R
G
G G
∀x∈R,∀α > 0, G ∩[x, x +α[6=∅
G0
α G ∩]0, α[ = ∅
Iα
2G∩I
G
G∩R∗
+m
G={km, k ∈Z}Zm
x y
X=Zx={kx, k ∈Z}, Y =Zy={ky, k ∈Z}, S ={mx +ny, (m, n)∈Z2}
X Y S (R,+) X
x Y y S
x y
Sx
y∈Q
x
yA B
A={kx, k ∈Z∗}, B ={ky, k ∈Z∗}
A∩B=∅
inf{|a−b|,(a, b)∈A×B}= 0
π
{cos n, n ∈Z}[−1,1]