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Number theory Difficulty 6.5 National olympiad Find the answer

Positive integers are put into the following table.

\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|}
\hline
1 & 3 & 6 & 10 & 15 & 21 & 28 & 36 & & \\ \hline
2 & 5 & 9 & 14 & 20 & 27 & 35 & 44 & & \\ \hline
4 & 8 & 13 & 19 & 26 & 34 & 43 & 53 & & \\ \hline
7 & 12 & 18 & 25 & 33 & 42 & & & & \\ \hline
11 & 17 & 24 & 32 & 41 & & & & & \\ \hline
16 & 23 & & & & & & & & \\ \hline
... & & & & & & & & & \\ \hline
... & & & & & & & & & \\ \hline
\end{tabular}

Find the number of the line and column where the number 20152015 stays.

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

Solution

We will analyze the pattern in the table to determine where the number 20152015 is located. The arrangement of numbers in the table has a specific formulation in terms of line and column indices. We will first observe the pattern, derive the general formula for numbers at position (i,j)(i, j), and then use it to find the location of the number 20152015.

1. Pattern Observation:

Looking at the first few values:
- First row: 1,3,6,10,15,1, 3, 6, 10, 15, \ldots (These are triangular numbers: Tn=n(n+1)2T_n = \frac{n(n+1)}{2})
- First column: 1,2,4,7,11,16,1, 2, 4, 7, 11, 16, \ldots (These numbers follow a pattern where the difference between terms increases by 1: (21),(42),(74),(2-1), (4-2), (7-4), \ldots)

2. Row Analysis:

The jj-th number in the first row is the jj-th triangular number:
Tj=j(j+1)2. T_j = \frac{j(j+1)}{2}.

3. Column Analysis:

The ii-th number in the first column is the sum of the first i1i-1 natural numbers plus 1:
Si=1+k=1i1k=1+(i1)i2. S_i = 1 + \sum_{k=1}^{i-1} k = 1 + \frac{(i-1)i}{2}.

4. General Formula:

For a number in the table located at position (i,j)(i, j), the formula is:
ai,j=Si+Tj1. a_{i,j} = S_i + T_{j-1}.
Simplified, this is:
ai,j=1+(i1)i2+(j1)j2. a_{i,j} = 1 + \frac{(i-1)i}{2} + \frac{(j-1)j}{2}.

5. **Solving for 20152015**:

We need to find ii and jj such that:
1+(i1)i2+(j1)j2=2015. 1 + \frac{(i-1)i}{2} + \frac{(j-1)j}{2} = 2015.

6. Approximations:

Start by trying values for ii:
- For i=62i = 62:
S62=1+(621)622=1+61622=1892. S_{62} = 1 + \frac{(62-1) \cdot 62}{2} = 1 + \frac{61 \cdot 62}{2} = 1892.
- Now solve:
1892+Tj1=2015Tj1=20151892=123. 1892 + T_{j-1} = 2015 \Rightarrow T_{j-1} = 2015 - 1892 = 123.
- Find jj:
(j1)j2=123(j1)j=246. \frac{(j-1)j}{2} = 123 \quad \Rightarrow \quad (j-1)j = 246.
- Solving j2j246=0j^2 - j - 246 = 0 using the quadratic formula yields:
j=1+1+42462=1+9852=1+312=16 j = \frac{1 + \sqrt{1 + 4 \cdot 246}}{2} = \frac{1 + \sqrt{985}}{2} = \frac{1 + 31}{2} = 16
However, this solution for (j1)=15(j-1) = 15; thus, j=2j = 2.

7. Conclusion:

The number 20152015 is located in line 62 and column 2 of the table. Therefore, the answer is:

(62,2) \boxed{(62, 2)}

This approach successfully determines the correct position of the number 20152015 in the table.

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