已知非常數的整係數多項式 f(x) 滿足 (x3+4x2+4x+3)f(x)=(x3−2x2+2x−1)f(x+1). 證明:對所有正整數 n (n≥8), f(n) 至少有五個不同的質因數。
Let f(x) be the polynomial with integer coefficients (f(x) is not constant) such that (x3+4x2+4x+3)f(x)=(x3−2x2+2x−1)f(x+1). Prove that for each positive integer n (n≥8), f(n) has at least five distinct prime divisors.
Solution
The given condition is equivalent to (x+3)(x2+x+1)f(x)=(x−1)(x2−x+1)f(x+1).(1) In equation (1), set x=−3,2−1−3i,2−1+3i,1 respectively, then f(−2)=f(21−3i)=f(21+3i)=f(1)=0. In equation (1), set x=−2,0. Then f(−1)=f(0)=0. Hence −2,−1,0,1 and 21±3i are roots of f(x)=0. Then f(x)=(x+2)(x+1)x(x−1)(x2−x+1)g(x),(2) where g(x) is a polynomial with real coefficients. From (2) we obtain f(x+1)=(x+3)(x+2)(x+1)x(x2+x+1)g(x+1).(3) Substituting (2), (3) into (1) gives g(x)=g(x+1). Let g(x)=∑k=0nakxk. Then ∑k=0nakxk=∑k=0nak(x+1)k. Comparing the coefficients of the (n−1)-th degree term on both sides, we know an−1=nan+an−1⇒nan=0. Therefore, g(x) is a constant c. Hence f(x)=c(x+2)(x+1)x(x−1)(x2−x+1), where the constant c is a nonzero integer.
First we prove: (n+2)(n+1)n(n−1) (n≥8) has at least four distinct prime divisors. Otherwise, (n+2)(n+1)n(n−1) has at most three distinct prime divisors 2,3,p (p=2,3). But since the greatest common divisors between (n−1),n,(n+1),(n+2) pairwise are 1,2,3, among the two odd numbers, which are coprime to each other, one is 3a and the other is pb, where a,b are positive integers. Consequently, the two even numbers are 2c+1,2×3d, where c,d are positive integers. Hence ∣2c−3d∣=1. Solving this gives (c,d)=(2,1),(3,2). Therefore, these two even numbers are 8, 6 or 16, 18. The former does not fit. The latter gives the other two odd numbers as 15, 17 or 17, 19, both of which lead to a contradiction.
Next, suppose there exists some positive integer n (n≥8) such that every prime divisor of n2−n+1 is also a prime divisor of (n+2)(n+1)n(n−1), and (n+2)(n+1)n(n−1) has exactly four prime divisors; otherwise, the conclusion holds. Clearly, (n2−n+1,n(n+1))=1. From n2−n+1=(n+2)(n−3)+7, we know (n2−n+1,n+1)=1 or 3,(n2−n+1,n+2)=1 or 7. Hence n2−n+1=3a7b, where a,b are 0 or positive integers. But 9∤(n2−n+1), so a∈{0,1}, and then b>0. By assumption, the prime divisors of n+2,n+1,n,n−1 are 2,3,7,p (p=2,3,7), so 7∣(n+2).
Consider the sets of prime divisors A,B of the two even numbers and the two odd numbers respectively among these. Clearly, 2∈A, ∣B∣≥2, A∩B⊆{3}. So ∣A∣=2 or ∣A∣=3 with 3∈A. If A={2,3} or {2,7}, then the two even numbers are 2c+1,2×3d or 2c+1,2×7d, giving ∣2c−3d∣=1 or ∣2c−7d∣=1. Hence these two even numbers are 16, 18 or 16, 14. The former gives 7∤(n+2); the latter makes (n+2)(n+1)n(n−1) have prime divisors 2,3,5,7 and 13 (or 17), a contradiction.
If A={2,p}, then n+2 is odd, and n−1 is even. From 3∈/A⇒3∤(n−1)⇒3∤(n−2). Hence n+2=7c,n=3d, and 2e∈{n+1,n−1} (c,d,e are 0 or positive integers, c,d≥2,e≥3). Consequently, ∣3d−2e∣=1⇒(d,e)=(2,3). Then n=9. Then n+2=11=7c, a contradiction.
If A={2,3,7}, then B={3,p}, and n+2 is even, (n+2,n−1)=3. Hence 2×3×7∣(n+2). Consequently, n=2c,n−1=3d,n+1=pe (c,d,e are positive integers, c≥3,d≥2). Then 2c−3d=1⇒(c,d)=(2,1), a contradiction.
If A={2,3,p}, then B={3,7}, and n+2 is odd, (n+2,n−1)=3. Hence 3×7∣(n+2). But (n,n+2)=1, so the odd prime divisors of n are not 3,7, a contradiction.
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