Problem:
Show that there are arbitrarily large numbers such that: (1) all its digits are 2 or more; and (2) the product of any four of its digits divides .
Solutions — 2
Solution 1
Solution:
and . So any number with as its last digits is divisible by . So consider . Its sum of digits is , so it is divisible by . Hence it is divisible by . But any four digits have at most four 's and at most two 's, so the product of any four digits divides and hence . But now we can extend by inserting an additional 's at the front. Its digit sum is increased by , so it remains divisible by and it is still divisible by the product of any four digits.
Solution 2
Solution:
The number with nine 's is divisible by . Hence the number with twenty-seven 's, which equals , is divisible by . So , the number with twenty-seven 's, is divisible by . Now the number with 's is divisible by and hence by .
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