To solve the problem of partitioning the numbers 20021,20031,…,20171 into two groups such that the absolute difference ∣A−B∣ is minimized, we can follow these steps:
1. Identify the sequence and its properties:
The sequence consists of the reciprocals of consecutive integers from 2002 to 2017. There are 16 terms in total.
2. Sum of the sequence:
Let S be the sum of all the terms in the sequence:
S=k=2002∑2017k1
3. Partition into two groups:
We need to partition the sequence into two groups, say X and Y, such that the sums of the reciprocals in each group are as close as possible. Let A be the sum of the reciprocals in group X and B be the sum of the reciprocals in group Y.
4. **Minimize ∣A−B∣:**
To minimize ∣A−B∣, we need to ensure that the sums A and B are as close as possible. Since A+B=S, we aim to have A≈B≈2S.
5. Consider partitions modulo 5:
To simplify the problem, consider the indices of the terms modulo 5. The indices 2002 to 2017 modulo 5 are:
2002≡2(mod5),2003≡3(mod5),2004≡4(mod5),2005≡0(mod5),2006≡1(mod5)
2007≡2(mod5),2008≡3(mod5),2009≡4(mod5),2010≡0(mod5),2011≡1(mod5)
2012≡2(mod5),2013≡3(mod5),2014≡4(mod5),2015≡0(mod5),2016≡1(mod5),2017≡2(mod5)
6. Count elements in each residue class:
Let xr denote the number of elements in X that are congruent to r(mod5), and similarly define yr for Y. We have:
x0+y0=4,x1+y1=3,x2+y2=4,x3+y3=3,x4+y4=2
7. Balance the sums:
To balance the sums, we need to distribute the elements such that the sums of the reciprocals are as close as possible. One possible way to achieve this is to ensure that the number of elements in each residue class is balanced between the two groups.
8. Construct the partition:
A possible partition that balances the sums is:
X={2005,2010,2015,2002,2007,2012,2003,2008,2013,2004,2009,2014}
Y={2006,2011,2016,2017}
This partition ensures that the sums A and B are as close as possible.
The final answer is X={2005,2010,2015,2002,2007,2012,2003,2008,2013,2004,2009,2014},Y={2006,2011,2016,2017}.