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Number theory Difficulty 4.7 AIME Prove it South Africa

Prove that there are infinitely many terms of the arithmetic sequence 1,14,27,40,,1+13k,1, 14, 27, 40, \dots, 1+13k, \dots which are of the form 22222222\ldots22. In other words a number that is made up using only the digit 22.

(Hint: 1001=7×11×131001 = 7 \times 11 \times 13)

Solution

Note that 221=13×17221 = 13 \times 17 and therefore 222=13×17+1222 = 13 \times 17 + 1 is in the arithmetic sequence. Furthermore, 222222221222\,222\,221 is divisible by 1313 and hence 222222222222\,222\,222 is in the sequence, since
222222=222×1001=222×7×11×13. 222222 = 222 \times 1001 = 222 \times 7 \times 11 \times 13.
In fact, any number consisting of 6m+36m + 3 22s will be one more than a multiple of 1313, since the number consisting of 6m6m 22s will be a multiple of 10011001 and hence a multiple of 1313. Thus there are infinitely many terms in the sequence that only uses the digit 22.

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