p(0)=−1, p(1)=1, p(2)=p(3)=−1, p(4)=7,
the roots of p are positive real numbers that belong to the union of the intervals (0,1), (1,2) and (3,4). Hence the roots are distinct, and neither of them is an integer. But since the coefficients of p are integers, neither can they be rational numbers, by the Gauss Lemma. Hence, u,v are irrational. Suppose now that u=vr, where r is rational, and r=1. Then v satisfies the pair of cubic equations:
v3−5v2+6v−1=0, r3v3−5r2v2+6rv−1=0.
Eliminating the term involving v3 in the usual way, we see that
(1−r)(5r2v2−6r(1+r)v+1+r+r2)=0.
Since r=1 by hypothesis, it follows that u satisfies the quadratic equation
5u2=6(r+1)u−(1+r+r2),
whence (multiplying by 5u and substituting for 5u2)
25u3=6(r+1)5u2−5(1+r+r2)u=(36(r+1)2−5(1+r+r2))u−6(r+1)(1+r+r2),
i.e.,
25u3=(31+67r+31r2)u−6(r+1)(1+r+r2).
Substituting these expressions for u3,u2 into u3−5u2+6u−1=0, we derive a linear equation for u with non-zero rational coefficients:
0=(31−83r+31r2)u−(1+r)(6−19r+6r2).
This implies that u is rational, which is false. This contradiction establishes the conclusion that u/v is also irrational.