A338625 a(1) = 1; for n > 1, ceiling(a(n-1)/2)+3 if a(n-1) and n are coprime, otherwise a(n) = GCD(a(n-1),n)*2+1.

1, 4, 5, 6, 6, 13, 10, 5, 6, 5, 6, 13, 27, 17, 12, 9, 8, 5, 6, 5, 6, 5, 6, 13, 10, 5, 6, 5, 6, 13, 10, 5, 6, 5, 11, 9, 8, 5, 6, 5, 6, 13, 10, 5, 11, 9, 8, 17, 12, 5, 6, 5, 6, 13, 10, 5, 6, 5, 6, 13, 10, 5, 6, 5, 11, 23, 15, 11, 9, 8, 7, 7, 7, 7, 7, 7, 15, 7

Offset

1,2

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

a(2) = 4. Because a(1) = 1 and 2 are coprime: ceiling(1/2)+3 = 4.

Mathematica

Nest[Append[#1, If[GCD[#1[[-1]], #2] == 1, Ceiling[#1[[-1]]/2] + 3, 2 GCD[#1[[-1]], #2] + 1]] & @@ {#, Length[#] + 1} &, {1}, 77] (* Michael De Vlieger, Nov 04 2020 *)
nxt[{n_, a_}]:={n+1, If[CoprimeQ[n+1, a], Ceiling[a/2]+3, 2*GCD[a, n+1]+1]}; NestList[nxt, {1, 1}, 80][[;; , 2]] (* Harvey P. Dale, Aug 27 2025 *)

Crossrefs

Sequence in context: A116962 A273777 A023846 * A046345 A325103 A004445

Adjacent sequences: A338622 A338623 A338624 * A338626 A338627 A338628

Keyword

nonn

Author

Noah Schlüssel, Nov 04 2020

Status

approved