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Number theory Difficulty 6.2 National olympiad Find the answer

Find all integers nn such that n4+8n+11n^{4}+8 n+11 is a product of two or more consecutive integers.

A number or a short expression. Spacing and $ signs are ignored.

Solutions — 2

Solution 1

We will prove that n4+8n+11n^{4}+8 n+11 is never a multiple of 3. This is clear if nn is a multiple of 3. If
nn is not a multiple of 3, then n4+8n+11=(n41)+12+8n=(n1)(n+1)(n2+1)+12+8nn^{4}+8 n+11=\left(n^{4}-1\right)+12+8 n=(n-1)(n+1)\left(n^{2}+1\right)+12+8 n, where 8n8 n is the only term not divisible by 3. Thus n4+8n+11n^{4}+8 n+11 is never the product of three or more integers.

It remains to discuss the case when n4+8n+11=y(y+1)n^{4}+8 n+11=y(y+1) for some integer yy. We write this as 4(n4+8n+11)=4y(y+1)4\left(n^{4}+8 n+11\right)=4 y(y+1) or 4n4+32n+45=(2y+1)24 n^{4}+32 n+45=(2 y+1)^{2}. A check shows that among n=±1n= \pm 1 and n=0n=0 only n=1n=1 satisfies the requirement, as 14+81+11=20=451^{4}+8 \cdot 1+11=20=4 \cdot 5. Now let n2|n| \geq 2. The identities 4n2+32n+45=(2n22)2+8(n+2)2+94 n^{2}+32 n+45=\left(2 n^{2}-2\right)^{2}+8(n+2)^{2}+9 and 4n4+32n+45=4 n^{4}+32 n+45= (2n2+8)232n(n1)19\left(2 n^{2}+8\right)^{2}-32 n(n-1)-19 indicate that for n2,2n22<2y+1<2n2+8|n| \geq 2,2 n^{2}-2<2 y+1<2 n^{2}+8. But 2y+12 y+1 is odd, so it can equal 2n2±1;2n2+3;2n2+52 n^{2} \pm 1 ; 2 n^{2}+3 ; 2 n^{2}+5 or 2n2+72 n^{2}+7. We investigate them one by one.

If 4n4+32n+45=(2n21)2n2+8n+11=0(n+4)2=54 n^{4}+32 n+45=\left(2 n^{2}-1\right)^{2} \Rightarrow n^{2}+8 n+11=0 \Rightarrow(n+4)^{2}=5, which is impossible, as 5 is not a perfect square.

If 4n4+32n+45=(2n2+1)2n28n11=0(n4)2=274 n^{4}+32 n+45=\left(2 n^{2}+1\right)^{2} \Rightarrow n^{2}-8 n-11=0 \Rightarrow(n-4)^{2}=27 which also fails.

Also 4n4+32n+45=(2n2+3)23n28n9=09n224n27=0(3n4)2=434 n^{4}+32 n+45=\left(2 n^{2}+3\right)^{2} \Rightarrow 3 n^{2}-8 n-9=0 \Rightarrow 9 n^{2}-24 n-27=0 \Rightarrow(3 n-4)^{2}=43 fails.

If 4n4+32n+45=(2n2+5)25n28n=525n240n=25(5n4)2=414 n^{4}+32 n+45=\left(2 n^{2}+5\right)^{2} \Rightarrow 5 n^{2}-8 n=5 \Rightarrow 25 n^{2}-40 n=25 \Rightarrow(5 n-4)^{2}=41 which also fails.

Finally, if 4n4+32n+45=(2n2+7)24 n^{4}+32 n+45=\left(2 n^{2}+7\right)^{2}, then 28n232n+4=04(n1)(7n1)=028 n^{2}-32 n+4=0 \Rightarrow 4(n-1)(7 n-1)=0, whence n=1n=1 that we already found. Thus the only solution is n=1n=1.

Solution 2

To solve the problem of finding all integers n n such that n4+8n+11 n^4 + 8n + 11 is a product of two or more consecutive integers, we can follow these steps:

1. Check for the product of three or more consecutive integers:
- If n4+8n+11 n^4 + 8n + 11 is a product of three or more consecutive integers, it must be divisible by 3.
- We check the expression modulo 3:
n4+8n+11(mod3) n^4 + 8n + 11 \pmod{3}
- For n0(mod3) n \equiv 0 \pmod{3} :
04+80+11112(mod3) 0^4 + 8 \cdot 0 + 11 \equiv 11 \equiv 2 \pmod{3}
- For n1(mod3) n \equiv 1 \pmod{3} :
14+81+111+8+11202(mod3) 1^4 + 8 \cdot 1 + 11 \equiv 1 + 8 + 11 \equiv 20 \equiv 2 \pmod{3}
- For n2(mod3) n \equiv 2 \pmod{3} :
24+82+1116+16+11431(mod3) 2^4 + 8 \cdot 2 + 11 \equiv 16 + 16 + 11 \equiv 43 \equiv 1 \pmod{3}
- Since n4+8n+112(mod3) n^4 + 8n + 11 \equiv 2 \pmod{3} or 1(mod3) 1 \pmod{3} , it is never divisible by 3. Therefore, n4+8n+11 n^4 + 8n + 11 cannot be a product of three or more consecutive integers.

2. Check for the product of two consecutive integers:
- Let n4+8n+11=a(a+1) n^4 + 8n + 11 = a(a+1) .
- This implies:
a2+a=n4+8n+11 a^2 + a = n^4 + 8n + 11
- Completing the square on the left-hand side:
a2+a+14=n4+8n+11+14 a^2 + a + \frac{1}{4} = n^4 + 8n + 11 + \frac{1}{4}
(a+12)2=n4+8n+11+14 \left(a + \frac{1}{2}\right)^2 = n^4 + 8n + 11 + \frac{1}{4}
(a+12)2=n4+8n+454 \left(a + \frac{1}{2}\right)^2 = n^4 + 8n + \frac{45}{4}
- Let b=a+12 b = a + \frac{1}{2} , then:
b2=n4+8n+454 b^2 = n^4 + 8n + \frac{45}{4}
4b2=4n4+32n+45 4b^2 = 4n^4 + 32n + 45
- We need 4n4+32n+45 4n^4 + 32n + 45 to be a perfect square. Let's check the range for n10 n \geq 10 :
(2n2)2<4n4+32n+45<(2n2+1)2 (2n^2)^2 < 4n^4 + 32n + 45 < (2n^2 + 1)^2
4n4<4n4+32n+45<4n4+4n2+1 4n^4 < 4n^4 + 32n + 45 < 4n^4 + 4n^2 + 1
- For n10 n \geq 10 , the inequality holds, so we only need to check values for n9 n \leq 9 .

3. **Check values for n9 n \leq 9 :**
- For n=0 n = 0 :
n4+8n+11=04+80+11=11(not a product of two consecutive integers) n^4 + 8n + 11 = 0^4 + 8 \cdot 0 + 11 = 11 \quad (\text{not a product of two consecutive integers})
- For n=1 n = 1 :
n4+8n+11=14+81+11=1+8+11=20=45(product of two consecutive integers) n^4 + 8n + 11 = 1^4 + 8 \cdot 1 + 11 = 1 + 8 + 11 = 20 = 4 \cdot 5 \quad (\text{product of two consecutive integers})
- For n=2 n = 2 :
n4+8n+11=24+82+11=16+16+11=43(not a product of two consecutive integers) n^4 + 8n + 11 = 2^4 + 8 \cdot 2 + 11 = 16 + 16 + 11 = 43 \quad (\text{not a product of two consecutive integers})
- For n=3 n = 3 :
n4+8n+11=34+83+11=81+24+11=116(not a product of two consecutive integers) n^4 + 8n + 11 = 3^4 + 8 \cdot 3 + 11 = 81 + 24 + 11 = 116 \quad (\text{not a product of two consecutive integers})
- For n=4 n = 4 :
n4+8n+11=44+84+11=256+32+11=299(not a product of two consecutive integers) n^4 + 8n + 11 = 4^4 + 8 \cdot 4 + 11 = 256 + 32 + 11 = 299 \quad (\text{not a product of two consecutive integers})
- For n=5 n = 5 :
n4+8n+11=54+85+11=625+40+11=676=2626(not a product of two consecutive integers) n^4 + 8n + 11 = 5^4 + 8 \cdot 5 + 11 = 625 + 40 + 11 = 676 = 26 \cdot 26 \quad (\text{not a product of two consecutive integers})
- For n=6 n = 6 :
n4+8n+11=64+86+11=1296+48+11=1355(not a product of two consecutive integers) n^4 + 8n + 11 = 6^4 + 8 \cdot 6 + 11 = 1296 + 48 + 11 = 1355 \quad (\text{not a product of two consecutive integers})
- For n=7 n = 7 :
n4+8n+11=74+87+11=2401+56+11=2468(not a product of two consecutive integers) n^4 + 8n + 11 = 7^4 + 8 \cdot 7 + 11 = 2401 + 56 + 11 = 2468 \quad (\text{not a product of two consecutive integers})
- For n=8 n = 8 :
n4+8n+11=84+88+11=4096+64+11=4171(not a product of two consecutive integers) n^4 + 8n + 11 = 8^4 + 8 \cdot 8 + 11 = 4096 + 64 + 11 = 4171 \quad (\text{not a product of two consecutive integers})
- For n=9 n = 9 :
n4+8n+11=94+89+11=6561+72+11=6644(not a product of two consecutive integers) n^4 + 8n + 11 = 9^4 + 8 \cdot 9 + 11 = 6561 + 72 + 11 = 6644 \quad (\text{not a product of two consecutive integers})

Conclusion:
The only integer n n such that n4+8n+11 n^4 + 8n + 11 is a product of two or more consecutive integers is n=1 n = 1 .

The final answer is n=1 \boxed{ n = 1 } .

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.