A106790 - OEIS

A106790

Smallest k>0 such that binomial(n,k) + 1 is prime.

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

1,3

COMMENTS

a(n) <= n; for primes p: a(p-1) = 1.

The values of n for which a(n)=n yield the sequence A067317. - Emeric Deutsch, Aug 27 2007

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

LINKS

Robert Israel, Table of n, a(n) for n = 1..4000

MAPLE

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

MATHEMATICA

a[n_] := For[k = 1, True, k++, If[PrimeQ[Binomial[n, k] + 1], Return[k]]];

Array[a, 70] (* Jean-François Alcover, Feb 13 2018 *)

CROSSREFS

Cf. A000040, A067317.

Sequence in context: A140185 A229341 A372245 * A078897 A322034 A351436

Adjacent sequences: A106787 A106788 A106789 * A106791 A106792 A106793

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, May 16 2005

EXTENSIONS

Corrected and extended by Emeric Deutsch, Aug 27 2007

STATUS

approved