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Algebra Difficulty 7.6 National olympiad, round 2 Find the answer

What is the smallest positive integer tt such that there exist integers x1,x2,,xtx_1,x_2,\ldots,x_t with x13+x23++xt3=20022002?x^3_1+x^3_2+\,\ldots\,+x^3_t=2002^{2002}\,?

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

Solution

To determine the smallest positive integer t t such that there exist integers x1,x2,,xt x_1, x_2, \ldots, x_t satisfying

x13+x23++xt3=20022002, x_1^3 + x_2^3 + \cdots + x_t^3 = 2002^{2002},

we will apply Fermat's Last Theorem and results regarding sums of cubes.

### Step 1: Understanding the Sum of Cubes
The problem requires expressing a large number, 20022002 2002^{2002} , as a sum of cubes. This can be directly related to a result in number theory: every integer can be expressed as the sum of four cubes. We need to determine if three cubes suffice or if four are necessary.

### Step 2: Evaluating Cubes and Powers
Calculate the properties of 20022002 2002^{2002} , and recognize:

- 20022(mod9)200224(mod9) 2002 \equiv 2 \pmod{9} \Rightarrow 2002^2 \equiv 4 \pmod{9} .
- 200238(mod9)200220028667×4(1)667×445(mod9) 2002^3 \equiv 8 \pmod{9} \Rightarrow 2002^{2002} \equiv 8^{667} \times 4 \equiv (-1)^{667} \times 4 \equiv -4 \equiv 5 \pmod{9} .

A cube modulo 9 can only be congruent to 0, 1, 8 after checking the possibilities for numbers from 0 to 8. Thus, a single cube cannot match 5(mod9) 5 \pmod{9} . Therefore, more than three cubes might be needed.

### Step 3: Constructing the Solution with t=4 t = 4
Given the difficulty ensuring 200220025(mod9) 2002^{2002} \equiv 5 \pmod{9} with three cubes and the result that four cubes are always sufficient, we reaffirm that there indeed exist integers x1,x2,x3,x4 x_1, x_2, x_3, x_4 such that:

x13+x23+x33+x43=20022002. x_1^3 + x_2^3 + x_3^3 + x_4^3 = 2002^{2002}.

While theoretically possible to attempt to prove with three cubes, doing so is difficult based on modular arithmetic properties shown, especially since directly proving three-cube sufficiency mathematically is complex without counterexample construction.

### Conclusion

Therefore, the smallest t t such that the sum of cubes equals 20022002 2002^{2002} is 4\boxed{4}.

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