x < y + 1 ⇔ x 6 y x < y ⇒ x + 1 6 y ¬x < 0
x + y = 0 ⇒ x = 0 ∧ y = 0
xy = 0 ⇒ x = 0 ∨ y = 0
xz = yz ∧ z 6= 0 ⇒ x = y
x < y ⇒ x + z < y + z
x < y ∧ z 6= 0 ⇒ xz < yz
A(x) A ` A(0)
¬X ⇒ (X ⇒ Y ) x + 1 = x + 1 ⇒ ∃y x + 1 = y + 1
A ` ∃y x + 1 = y + 1 A ` A(x) ⇒ A(x + 1)
A ` ∀xA
A ` 0 + x = x 0 + 0 = 0
0 + (x + 1) = (0 + x) + 1 0 + x = x
0+(x+1) = x+1 0+x = 0 ⇒ 0+(x+1) =
0 0 + 0 = 0 ∧∀x(0 + x = 0 ⇒ 0 + (x + 1) = x + 1) ⇒
∀x 0 + x = x A ` 0 + x = x
(x+1)+y = (x+y)+1 y = 0
(x + 1) + 0 = x + 1 = (x + 0) + 1 (x + 1) + y = (x + y) + 1
(x+1)+(y+1) = ((x+1)+y)+1 = ((x+y)+1)+1 =
(x + (y + 1)) + 1 A ` (x + 1) + y = (x + y) + 1
x + y = y + x (x + 1) + y = (x + y) + 1 =
(y + x) + 1 = y + (x + 1) A ` x + y = y + x
(x + y) + 0 = x + y = x + (y + 0) (x + y) + z = x + (y + z)
(x + y) + (z + 1) = ((x + y) + z) + 1 = (x + (y + z)) + 1 = x + ((y + z) + 1) =
x + (y + (z + 1)) A ` (x + y) + z = x + (y + z)
x(y + 0) = xy = xy + 0 = xy + x0 x(y + z) = xy + xz
x(y + (z + 1)) = x((y + z) + 1) = x(y + z ) + x = (xy + xz) + x =
xy + (xz + x) = xy + x(z + 1) A ` x(y + z) = xy +xz
0 · 0 = 0 0x = 0 0(x + 1) = 0x + 0 = 0 + 0 = 0
A ` 0x = 0 = x0 A ` (x + 1)y = yx + y = y(x + 1)
y = 0 (x + 1) ·0 = 0 = x ·0 + 0 (x + 1)y = xy + y
(x + 1)(y + 1) = (x + 1)y + (x + 1) = (xy + y) + (x + 1) = (xy + x) + (y + 1) =
x(y + 1) + (y + 1) A ` (x + 1)y = xy + y
A ` xy = yx
(xy)(zt) x +
((y + z) + t) xyzt x + y + z + t
z = 0 x+z = y+z ⇒ x = y
x+(z+1) = y+(z+1) (x+z)+1 = (y+z)+1 x+z = y+z
x = y A ` x + z = y + z ⇒ x = y
0 6 y y = y + 0 = 0 + y y 6 x x = y + z
x + 1 = y + z + 1 y 6 x + 1 x 6 y y = x + z
z 6= 0 A ` ∃z z = z + 1 c z = c + 1
y = x + (c + 1) = (x + 1) + c y 6 x + 1 z = 0 y = x 6 x + 1