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Combinatorics Difficulty 6.8 National olympiad Find the answer

The numbers 1,2,...,101, 2, . . . , 10 are randomly arranged in a circle. Let pp be the probability that for every positive integer k<10k < 10, there exists an integer k>kk' > k such that there is at most one number between kk and kk' in the circle. If pp can be expressed as ab\frac{a}{b} for relatively prime positive integers aa and bb, compute 100a+b100a + b.

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

Solution

To solve the problem, we need to find the probability p p that for every positive integer k<10 k < 10 , there exists an integer k>k k' > k such that there is at most one number between k k and k k' in the circle. We will use a recursive approach to determine this probability.

1. Define the Recursive Function:
Let an a_n be the number of ways to permute 1,2,3,,n 1, 2, 3, \ldots, n cyclically such that the given condition holds. We call a number a a "close enough" to b b if there is at most one number between a a and b b .

2. Base Cases:
For small values of n n , we can manually verify the permutations:
- a4=1 a_4 = 1
- a5=1 a_5 = 1

3. Recursive Formula:
We consider two cases to develop the recursive formula for an a_n :
- Case 1: n n and n1 n-1 are adjacent. Removing n n from the circle leaves a valid permutation of n1 n-1 numbers. Thus, there are 2an1 2a_{n-1} ways to insert n n adjacent to n1 n-1 .
- Case 2: n n and n1 n-1 have one number between them. Removing both n n and the number between them leaves a valid permutation of n2 n-2 numbers. There are 2(n2)an2 2(n-2)a_{n-2} ways to insert n n and n1 n-1 with one number between them.

Combining these cases, we get the recursive relation:
an=2an1+2(n2)an2 a_n = 2a_{n-1} + 2(n-2)a_{n-2}

4. Probability Calculation:
Let pn=an(n1)! p_n = \frac{a_n}{(n-1)!} . Then:
pn(n1)!=2pn1(n2)!+2pn2(n2)! p_n(n-1)! = 2p_{n-1}(n-2)! + 2p_{n-2}(n-2)!
Simplifying, we get:
pn=2n1(pn1+pn2) p_n = \frac{2}{n-1}(p_{n-1} + p_{n-2})

5. **Compute p10 p_{10} :**
Using the base cases p4=1 p_4 = 1 and p5=1 p_5 = 1 , we compute the subsequent values:
p6=25(p5+p4)=25(1+1)=45 p_6 = \frac{2}{5}(p_5 + p_4) = \frac{2}{5}(1 + 1) = \frac{4}{5}
p7=26(p6+p5)=26(45+1)=2695=35 p_7 = \frac{2}{6}(p_6 + p_5) = \frac{2}{6}\left(\frac{4}{5} + 1\right) = \frac{2}{6} \cdot \frac{9}{5} = \frac{3}{5}
p8=27(p7+p6)=27(35+45)=2775=25 p_8 = \frac{2}{7}(p_7 + p_6) = \frac{2}{7}\left(\frac{3}{5} + \frac{4}{5}\right) = \frac{2}{7} \cdot \frac{7}{5} = \frac{2}{5}
p9=28(p8+p7)=28(25+35)=281=14 p_9 = \frac{2}{8}(p_8 + p_7) = \frac{2}{8}\left(\frac{2}{5} + \frac{3}{5}\right) = \frac{2}{8} \cdot 1 = \frac{1}{4}
p10=29(p9+p8)=29(14+25)=291320=1390 p_{10} = \frac{2}{9}(p_9 + p_8) = \frac{2}{9}\left(\frac{1}{4} + \frac{2}{5}\right) = \frac{2}{9} \cdot \frac{13}{20} = \frac{13}{90}

6. Final Calculation:
The probability p p is 1390 \frac{13}{90} . To find 100a+b 100a + b where p=ab p = \frac{a}{b} and a a and b b are relatively prime, we have:
100a+b=10013+90=1300+90=1390 100a + b = 100 \cdot 13 + 90 = 1300 + 90 = 1390

The final answer is 1390 \boxed{1390}

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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.