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A106790
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Smallest k>0 such that binomial(n,k) + 1 is prime.
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1
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1, 1, 3, 1, 2, 1, 7, 2, 2, 1, 4, 1, 2, 5, 15, 1, 2, 1, 4, 2, 2, 1, 23, 2, 6, 4, 4, 1, 7, 1, 31, 10, 6, 10, 4, 1, 18, 15, 9, 1, 2, 1, 17, 2, 2, 1, 23, 2, 4, 20, 6, 1, 6, 8, 10, 6, 2, 1, 59, 1, 2, 25, 63, 2, 2, 1, 67, 8, 2, 1
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OFFSET
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1,3
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COMMENTS
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a(n) <= n; for primes p: a(p-1) = 1.
If a(n) > n/2 then a(n) = n. a(n) = floor(n/2) for n = 2, 5, 37, 47, 124. Are there others? - Robert Israel, Mar 09 2020
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LINKS
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MAPLE
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a:=proc(n) local k: for k while isprime(1+binomial(n, k))=false do end do: k end proc: seq(a(n), n=1..70); # Emeric Deutsch, Aug 27 2007
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MATHEMATICA
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a[n_] := For[k = 1, True, k++, If[PrimeQ[Binomial[n, k] + 1], Return[k]]];
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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