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Number theory Difficulty 6.2 National Olympiad Prove it Philippines

Problem:
Let SS be a subset of {1,2,,2017}\{1,2, \ldots, 2017\} such that no two elements of SS have a sum divisible by 3737. Find the maximum number of elements that SS can have.

Solution

Solution:
Let us consider the set {1,2,,2017}\{1,2,\ldots,2017\} modulo 3737. Since 2017=37×54+192017 = 37 \times 54 + 19, the residues modulo 3737 that appear are 1,2,,371,2,\ldots,37 repeated 5454 times each, and 1,2,,191,2,\ldots,19 appearing one more time each.

Let us group the numbers by their residues modulo 3737.

For each pair of residues rr and 37r37 - r (with 1r181 \leq r \leq 18), the sum r+(37r)=37r + (37 - r) = 37 is divisible by 3737. Thus, we cannot have both rr and 37r37 - r in SS.

The residue 00 (i.e., multiples of 3737) is its own pair, so at most one element with residue 00 can be in SS.

If 3737 is even, then r=37/2=18.5r = 37/2 = 18.5, but since 3737 is odd, there is no such residue.

So, for each pair {r,37r}\{r, 37 - r\} with 1r181 \leq r \leq 18, we can take all numbers with residue rr or all with residue 37r37 - r, but not both.

Let us count how many numbers there are for each residue:
- For 1r191 \leq r \leq 19, there are 5555 numbers with residue rr (since 5454 full cycles plus one extra for 1r191 \leq r \leq 19).
- For 20r3720 \leq r \leq 37, there are 5454 numbers with residue rr.

Let us maximize the total size of SS.

For each pair {r,37r}\{r, 37 - r\} with 1r181 \leq r \leq 18:
- rr has 5555 numbers if 1r181 \leq r \leq 18.
- 37r37 - r has 5454 numbers if 1937r3619 \leq 37 - r \leq 36 (i.e., 1r181 \leq r \leq 18).

So, for each pair, we should pick the residue with more numbers, i.e., rr (1r181 \leq r \leq 18), which has 5555 numbers.

There are 1818 such pairs, so 18×55=99018 \times 55 = 990 numbers.

Now, consider residue 00 (i.e., 37,74,,201737, 74, \ldots, 2017):
- The numbers with residue 00 are 37,74,,199837, 74, \ldots, 1998.
- 2017÷37=54.52017 \div 37 = 54.5, so 5454 multiples of 3737 up to 19981998.
- 37×55=2035>201737 \times 55 = 2035 > 2017, so only 5454 such numbers.
- So, at most one can be chosen.

Residue 1919 is left unpaired (since 3719=1837 - 19 = 18 is already paired with 1919). Residue 1919 has 5555 numbers.

Thus, the maximum is:
18×55+55+1=990+55+1=1046. 18 \times 55 + 55 + 1 = 990 + 55 + 1 = 1046.

Therefore, the answer is 1046\boxed{1046}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.