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, 2022

Algebra Difficulty 6.1 National Olympiad Prove it Japan

Find the number of tuples of positive integers (a1,a2,,a2022)(a_1, a_2, \dots, a_{2022}) which satisfy a1<a2<<a2022a_1 < a_2 < \dots < a_{2022} and
a1262a2272a2022220272. a_1^2 - 6^2 \ge a_2^2 - 7^2 \ge \dots \ge a_{2022}^2 - 2027^2.

Solution

Since 1a1<a2<<a20221 \le a_1 < a_2 < \dots < a_{2022}, we have aiia_i \ge i for all 1i20221 \le i \le 2022. Also, the condition a1262a2272a2022220272a_1^2 - 6^2 \ge a_2^2 - 7^2 \ge \dots \ge a_{2022}^2 - 2027^2 is equivalent to ai+12ai2(i+6)2(i+5)2=2i+11a_{i+1}^2 - a_i^2 \le (i+6)^2 - (i+5)^2 = 2i + 11 (1i20211 \le i \le 2021).

When ai+1ai+3a_{i+1} \ge a_i + 3 holds for some 1i20211 \le i \le 2021, we have ai+12ai2(ai+3)2ai2=6ai+96i+9>2i+11a_{i+1}^2 - a_i^2 \ge (a_i + 3)^2 - a_i^2 = 6a_i + 9 \ge 6i + 9 > 2i + 11 for such ii, which contradicts to ai+12ai22i+11a_{i+1}^2 - a_i^2 \le 2i + 11. Therefore, since ai<ai+1a_i < a_{i+1}, we have ai+1=ai+1a_{i+1} = a_i + 1 or ai+1=ai+2a_{i+1} = a_i + 2 (1i20211 \le \forall i \le 2021).
For each ii, if ai+1=ai+1a_{i+1} = a_i + 1, then since ai+12ai2=2ai+1a_{i+1}^2 - a_i^2 = 2a_i + 1, ai+12ai22i+11a_{i+1}^2 - a_i^2 \le 2i + 11 is equivalent to aii+5a_i \le i + 5. Therefore, the tuples satisfying ai+1=ai+1a_{i+1} = a_i + 1 for all 1i20211 \le i \le 2021 are of the form ai=i+ca_i = i + c for a constant integer 0c50 \le c \le 5. There are exactly six such tuples.
Hereafter, we assume that ai+1=ai+2a_{i+1} = a_i + 2 for some 1i20211 \le i \le 2021 and denote the maximum of such ii by jj. Since 4j+44aj+4=aj+12aj22j+114j + 4 \le 4a_j + 4 = a_{j+1}^2 - a_j^2 \le 2j + 11, we have j=1,2,3j = 1, 2, 3.
* When j=1j = 1, since 4a1+4=a22a1221+114a_1 + 4 = a_2^2 - a_1^2 \le 2 \cdot 1 + 11, we have a1=1,2a_1 = 1, 2. In this case, (a1,a2,,a2022)=(1,3,4,5,,2022,2023)(a_1, a_2, \dots, a_{2022}) = (1, 3, 4, 5, \dots, 2022, 2023), (2,4,5,6,,2023,2024)(2, 4, 5, 6, \dots, 2023, 2024).
* When j=2j = 2, since 4a2+4=a32a2222+114a_2 + 4 = a_3^2 - a_2^2 \le 2 \cdot 2 + 11 and a22a_2 \ge 2, we have a2=2a_2 = 2. In this case, (a1,a2,,a2022)=(1,2,4,5,6,,2022,2023)(a_1, a_2, \dots, a_{2022}) = (1, 2, 4, 5, 6, \dots, 2022, 2023).
* When j=3j = 3, since 4a3+4=a42a3223+114a_3 + 4 = a_4^2 - a_3^2 \le 2 \cdot 3 + 11 and a33a_3 \ge 3, we have a3=3a_3 = 3. In this case, (a1,a2,,a2022)=(1,2,3,5,6,7,,2022,2023)(a_1, a_2, \dots, a_{2022}) = (1, 2, 3, 5, 6, 7, \dots, 2022, 2023).
Hence the total number of the tuples is 6+4=106 + 4 = 10.

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