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Combinatorics Difficulty 7.4 National olympiad, round 2 Find the answer

Let [a][a] represent the largest integer less than or equal to aa, for any real number aa. Let {a}=a[a]\{a\} = a - [a].

Are there positive integers m,nm,n and n+1n+1 real numbers x0,x1,\hdots,xnx_0,x_1,\hdots,x_n such that x0=428x_0=428, xn=1928x_n=1928, xk+110=[xk10]+m+{xk5}\frac{x_{k+1}}{10} = \left[\frac{x_k}{10}\right] + m + \left\{\frac{x_k}{5}\right\} holds?

Justify your answer.

Solution

1. Define the problem and initial conditions:
We are given the function definitions:
[a] represents the largest integer less than or equal to a, [a] \text{ represents the largest integer less than or equal to } a,
{a}=a[a]. \{a\} = a - [a].
We need to determine if there exist positive integers mm and nn and n+1n+1 real numbers x0,x1,,xnx_0, x_1, \ldots, x_n such that:
x0=428,xn=1928, x_0 = 428, \quad x_n = 1928,
and
xk+110=[xk10]+m+{xk5}. \frac{x_{k+1}}{10} = \left[\frac{x_k}{10}\right] + m + \left\{\frac{x_k}{5}\right\}.

2. **Calculate the first few values of xix_i:**
x0=428. x_0 = 428.
Using the given recurrence relation:
x110=[42810]+m+{4285}. \frac{x_1}{10} = \left[\frac{428}{10}\right] + m + \left\{\frac{428}{5}\right\}.
Calculate each term:
[42810]=42, \left[\frac{428}{10}\right] = 42,
{4285}=4285[4285]=85.685=0.6. \left\{\frac{428}{5}\right\} = \frac{428}{5} - \left[\frac{428}{5}\right] = 85.6 - 85 = 0.6.
Therefore:
x110=42+m+0.6=42.6+m, \frac{x_1}{10} = 42 + m + 0.6 = 42.6 + m,
x1=10(42.6+m)=426+10m. x_1 = 10(42.6 + m) = 426 + 10m.

3. **Calculate x2x_2:**
x210=[x110]+m+{x15}. \frac{x_2}{10} = \left[\frac{x_1}{10}\right] + m + \left\{\frac{x_1}{5}\right\}.
Since x1=426+10mx_1 = 426 + 10m:
[426+10m10]=42+m, \left[\frac{426 + 10m}{10}\right] = 42 + m,
{426+10m5}=426+10m5[426+10m5]=85.2+2m(85+2m)=0.2. \left\{\frac{426 + 10m}{5}\right\} = \frac{426 + 10m}{5} - \left[\frac{426 + 10m}{5}\right] = 85.2 + 2m - (85 + 2m) = 0.2.
Therefore:
x210=42+m+m+0.2=42+2m+0.2, \frac{x_2}{10} = 42 + m + m + 0.2 = 42 + 2m + 0.2,
x2=10(42+2m+0.2)=422+20m. x_2 = 10(42 + 2m + 0.2) = 422 + 20m.

4. **Calculate x3x_3:**
x310=[x210]+m+{x25}. \frac{x_3}{10} = \left[\frac{x_2}{10}\right] + m + \left\{\frac{x_2}{5}\right\}.
Since x2=422+20mx_2 = 422 + 20m:
[422+20m10]=42+2m, \left[\frac{422 + 20m}{10}\right] = 42 + 2m,
{422+20m5}=422+20m5[422+20m5]=84.4+4m(84+4m)=0.4. \left\{\frac{422 + 20m}{5}\right\} = \frac{422 + 20m}{5} - \left[\frac{422 + 20m}{5}\right] = 84.4 + 4m - (84 + 4m) = 0.4.
Therefore:
x310=42+2m+m+0.4=42+3m+0.4, \frac{x_3}{10} = 42 + 2m + m + 0.4 = 42 + 3m + 0.4,
x3=10(42+3m+0.4)=424+30m. x_3 = 10(42 + 3m + 0.4) = 424 + 30m.

5. **Calculate x4x_4:**
x410=[x310]+m+{x35}. \frac{x_4}{10} = \left[\frac{x_3}{10}\right] + m + \left\{\frac{x_3}{5}\right\}.
Since x3=424+30mx_3 = 424 + 30m:
[424+30m10]=42+3m, \left[\frac{424 + 30m}{10}\right] = 42 + 3m,
{424+30m5}=424+30m5[424+30m5]=84.8+6m(84+6m)=0.8. \left\{\frac{424 + 30m}{5}\right\} = \frac{424 + 30m}{5} - \left[\frac{424 + 30m}{5}\right] = 84.8 + 6m - (84 + 6m) = 0.8.
Therefore:
x410=42+3m+m+0.8=42+4m+0.8, \frac{x_4}{10} = 42 + 3m + m + 0.8 = 42 + 4m + 0.8,
x4=10(42+4m+0.8)=428+40m. x_4 = 10(42 + 4m + 0.8) = 428 + 40m.

6. Analyze the periodic behavior:
Notice that:
x16(mod10), x_1 \equiv 6 \pmod{10},
x22(mod10), x_2 \equiv 2 \pmod{10},
x34(mod10), x_3 \equiv 4 \pmod{10},
x48(mod10). x_4 \equiv 8 \pmod{10}.
This pattern repeats every 4 steps. Therefore, for xn=1928x_n = 1928, we must have:
x4k=1928. x_{4k} = 1928.
Given:
x4k=428+40mk=1928, x_{4k} = 428 + 40mk = 1928,
40mk=1500. 40mk = 1500.
Since 1500 is not divisible by 40, there are no positive integers mm and kk that satisfy this equation.

The final answer is False.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.