Proof 1. By the previous Proposition, d(x) =
x(xd(x)). Then d(x)x =0. Thus d(x) ≤ x.
2. d(xy) = xd(y) ∧ d(x)y = (d(x)y)
((d(x)y)(xd(y))), then d(xy)(d(x)y) = 0. Thus d(xy)
≤ d(x)y.
3. d(xy) = d(x)y ∧ xd(y) = (xd(y))
((xd(y))(d(x)y)). Thus d(xy) ≤ xd(y).
4. d(xd(x)) = xd(d(x)) ∧ d(x)d(x) =
xd(d(x)) ∧ 0 = 0(0(xd(d(x)))) = 0
5. d(d(x)) = d(x(xd(x))) =
d(x)(xd(x)) ∧ xd(xd(x)) = d(x)(xd(x)) ∧ x =
x(x(d(x)(xd(x)))) Thus d(d(x)) ≤ x.
6. Since d is regular, d-1(0) ≠ φ. Let x, y