Does there exist a polynomial P(x) with integral coefficients such that
1. P(325+35)=220325+28435 ?
2. P(325+35)=1184325+121035 ?
Solution
First, we shall prove two following lemmas:
Lemma 1. If a is an integer number that is not a perfect cube, and m,n,p are integer numbers such that m+n3a+p3a2=0, then m=n=p=0. Proof of Lemma 1. Since m+n3a+p3a2=0, then 3a is the root of P(x)=px2+nx+m with integer coefficients. Consider the polynomial Q(x)=x3−a, then 3a is also a root of Q(x). Assume that Q(x) is reducible, then one of the factors must have degree 1, that is Q(x) has a rational root. But we know that 3a is irrational since a is not a perfect cube (by assumption). Hence, Q(x) is irreducible. In other words, Q(x) is the minimal polynomial of 3a, and this leads to P(x)=0, or m=n=p=0.
Lemma 2. Given that P(x) is a polynomial with integer coefficients, and a is an integer such that 3a is not an integer. If P(3a2+3a)=m+n3a+p3a2 then a−1∣n−p.
Proof of Lemma 2. We will prove by induction on the degree of P(x) which is denoted by k. For k=1, it is trivial. Assume that the statement is true for k=t, consider P(x) of degree t+1, we write P(x)=xQ(x)+s, with Q(x) of degree t and s=P(0) is an integer. Assume that Q(3a2+3a)=d+e3a+f3a2, then by induction hypothesis a−1∣e−f. This gives P(3a2+3a)=(3a2+3a)(d+e3a+f3a2)+s=s+ea+fa+(d+fa)3a+(d+e)3a2 Hence, (d+fa)−(d+e)=fa−e=(fa−f)+(f−e), which is a multiple of a−1.
Now we turn to the problem.
For part a), we show that P(x) does exist, for example we choose P(x)=ux2+vx+w, where u,v,w must satisfy u+v=220,5u+v=284,10u+w=0. That is, P(x)=16x2+204x−160.
For part b), since 1210−1184=26, which is not a multiple of 4, in this case there is no P(x) satisfying the condition of the problem.
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