Maths Olympiad Prep

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Number theory Difficulty 5.1 AIME, harder Prove it Ibero-American Mathematical Olympiad

Problem:
Show that there are arbitrarily large numbers nn such that: (1) all its digits are 2 or more; and (2) the product of any four of its digits divides nn.

Solutions — 2

Solution 1

Solution:
3232=16×2023232 = 16 \times 202 and 10000=16×62510000 = 16 \times 625. So any number with 32323232 as its last 44 digits is divisible by 1616. So consider N=22223232N = 22223232. Its sum of digits is 1818, so it is divisible by 99. Hence it is divisible by 916=1449 \cdot 16 = 144. But any four digits have at most four 22's and at most two 33's, so the product of any four digits divides 144144 and hence NN. But now we can extend NN by inserting an additional 9m9m 22's at the front. Its digit sum is increased by 18m18m, so it remains divisible by 144144 and it is still divisible by the product of any four digits.

Solution 2

Solution:
The number 111111111111111111 with nine 11's is divisible by 99. Hence the number with twenty-seven 11's, which equals 111111111×1000000001000000001111111111 \times 1000000001000000001, is divisible by 2727. So NN, the number with twenty-seven 33's, is divisible by 343^{4}. Now the number with 27n27n 33's is divisible by NN and hence by 343^{4}.

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