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Difficulty: 5/102023 IOQM 2023 (Q29)

A positive integer > 1 is called beautiful if can be written in one and only one way as = a1 + a2 + … + ak = a1  a2 … ak for some positive integers a1, a2, …., ak, where > 1 and a1  a2  ….  ak. (For example 6 is beautiful since 6 = 3  2  1 = 3 + 2 + 1; and this is unique. But 8 is not beautiful since 8 = 4 + 2 + 1 + 1 = 4  2  1  1 as well as 8 = 2 + 2 + 2 + 1 + 1 = 2  2  2  1  1, so uniqueness is lost.) Find the largest beautiful number less than 100.

Guide / Hint

Hint 1: Start by analyzing the initial conditions and setting up the basic equations. 99 = 9 × 11 × 1 × 1 × 1 × 1 ……………. × 1 (79 times ‘1’).

Hint 2: Look for algebraic properties, symmetry, or geometric theorems to simplify. = 9 + 11 + 1 + 1 + ----------- 79 times.

Hint 3: Proceed with the final algebraic steps to solve the system. solve for the final value × 3 × 1 × 1 × 1 ….. (66 times 1).

Solution

Step 1: 99 = 9 × 11 × 1 × 1 × 1 × 1 ……………. × 1 (79 times ‘1’)

Step 2: = 9 + 11 + 1 + 1 + ----------- 79 times

Step 3: = 33 × 3 × 1 × 1 × 1 ….. (66 times 1)

Step 4: = 33 + 3 + 1 + 1 + ……… 66 times

Step 5: = Hence 99 is not beautiful.

Step 6: 98 = 49 × 2 × 1 × 1 × ……. 47 times

Step 7: = 49 + 2 + 1 + 1 + …….. 47 times

Step 8: = 14 × 7 × 1 × 1 × …… 77 times

Step 9: = 14 + 7 + 1 + 1 + ……. 77 times

Step 10: Hence 98 is not beautiful.

Step 11: 97 is not beautiful as it can not be written in these expanded forms.

Step 12: 96 = 2 × 48 × 1 × 1 × ….. 46 times

Step 13: = 2 + 48 + 1 + 1 + ……. 46 times

Step 14: = 3 × 32 × 1 × 1 × ……. 61 times

Step 15: = 3 + 32 + 1 + 1 + …… 61 times

Step 16: Hence 96 is not beautiful.

Step 17: 95 = 19 × 5 × 1 × 1 × ….. 71 times

Step 18: = 19 + 5 + 1 + 1 + ……. 71 times

Step 19: As 95 is uniquely represents hence it is beautiful.

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    2023 IOQM 2023 Q29 - Olympiad Math Olympiad Question | Leminno