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Algebra Difficulty 7.2 National olympiad, round 2 Prove it

The sequence (an)(a_n) is defined by a1=0a_1=0,
an+1=a1+a2++ann+1. a_{n+1}={a_1+a_2+\ldots+a_n\over n}+1.
Prove that a2016>12+a1000a_{2016}>{1\over 2}+a_{1000}.

Solution

To prove that a2016>12+a1000 a_{2016} > \frac{1}{2} + a_{1000} , we need to analyze the given sequence and its properties.

1. Define the sequence and initial condition:
a1=0 a_1 = 0
an+1=a1+a2++ann+1 a_{n+1} = \frac{a_1 + a_2 + \ldots + a_n}{n} + 1

2. Calculate the first few terms to identify a pattern:
a2=a11+1=0+1=1 a_2 = \frac{a_1}{1} + 1 = 0 + 1 = 1
a3=a1+a22+1=0+12+1=12+1=32 a_3 = \frac{a_1 + a_2}{2} + 1 = \frac{0 + 1}{2} + 1 = \frac{1}{2} + 1 = \frac{3}{2}
a4=a1+a2+a33+1=0+1+323+1=523+1=56+1=116 a_4 = \frac{a_1 + a_2 + a_3}{3} + 1 = \frac{0 + 1 + \frac{3}{2}}{3} + 1 = \frac{\frac{5}{2}}{3} + 1 = \frac{5}{6} + 1 = \frac{11}{6}

3. Generalize the sequence:
Notice that the sequence ana_n can be expressed in terms of the harmonic series. We can rewrite the recurrence relation as:
Sn=a1+a2++an S_n = a_1 + a_2 + \ldots + a_n
an+1=Snn+1 a_{n+1} = \frac{S_n}{n} + 1
Sn+1=Sn+an+1=Sn+Snn+1=Sn(1+1n)+1 S_{n+1} = S_n + a_{n+1} = S_n + \frac{S_n}{n} + 1 = S_n \left(1 + \frac{1}{n}\right) + 1
Sn+1=Snn+1n+1 S_{n+1} = S_n \frac{n+1}{n} + 1

4. **Simplify the sum SnS_n:**
Sn+1=Snn+1n+1 S_{n+1} = S_n \frac{n+1}{n} + 1
Sn+1=Sn(1+1n)+1 S_{n+1} = S_n \left(1 + \frac{1}{n}\right) + 1
By induction, we can show that SnS_n grows approximately as nlognn \log n.

5. **Estimate ana_n:**
Since an=Sn1n1+1a_n = \frac{S_{n-1}}{n-1} + 1, we can approximate:
anlogn+1 a_n \approx \log n + 1

6. **Compare a2016a_{2016} and a1000a_{1000}:**
a2016log2016+1 a_{2016} \approx \log 2016 + 1
a1000log1000+1 a_{1000} \approx \log 1000 + 1
log2016log(2×1008)log2+log10080.301+log1000+log1.008 \log 2016 \approx \log (2 \times 1008) \approx \log 2 + \log 1008 \approx 0.301 + \log 1000 + \log 1.008
log1000=3,log1.0080.003 \log 1000 = 3, \quad \log 1.008 \approx 0.003
log20160.301+3+0.003=3.304 \log 2016 \approx 0.301 + 3 + 0.003 = 3.304
a20163.304+1=4.304 a_{2016} \approx 3.304 + 1 = 4.304
a10003+1=4 a_{1000} \approx 3 + 1 = 4
12+a1000=12+4=4.5 \frac{1}{2} + a_{1000} = \frac{1}{2} + 4 = 4.5

7. Conclusion:
a20164.304<4.5 a_{2016} \approx 4.304 < 4.5
Therefore, the inequality a2016>12+a1000a_{2016} > \frac{1}{2} + a_{1000} does not hold.

The final answer is a2016<12+a1000 \boxed{ a_{2016} < \frac{1}{2} + a_{1000} }

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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.