A270645 The sequence a of 1's and 2's starting with (2,1,1,1) such that a(n) is the length of the (n+2)nd run of a.

2, 1, 1, 1, 2, 2, 1, 2, 1, 2, 2, 1, 1, 2, 1, 1, 2, 1, 1, 2, 2, 1, 2, 1, 1, 2, 1, 2, 2, 1, 2, 1, 1, 2, 2, 1, 2, 2, 1, 2, 1, 1, 2, 1, 1, 2, 2, 1, 2, 2, 1, 2, 1, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 1, 2, 1, 2, 2, 1, 2, 1, 1, 2, 2, 1, 2, 2, 1, 1, 2, 1, 1, 2, 1, 2, 2

Offset: 1

Author

Clark Kimberling, Apr 06 2016

Keywords

• nonn
• easy

Comments

See A270641 for a guide to related sequences.

Examples

			a(1) = 1, so the 3rd run has length 2, so a(5) must be 2 and a(6) = 2.
a(2) = 2, so the 4th run has length 1, so a(7) = 1 and a(8) = 2.
a(3) = 1, so the 5th run has length 1, so a(9) = 1 and a(10) = 2.
Globally, the runlength sequence is 1,3,2,1,1,1,2,2,1,2,1,2,2,1,1,..., and deleting the first 2 terms leaves the same sequence.
		

Links

• Clark Kimberling, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000002, A006928, A022300, A270641.

Programs

Mathematica

a = {2,1,1,1}; Do[a = Join[a, ConstantArray[If[Last[a] == 1, 2, 1], {a[[n]]}]], {n,   200}]; a  (* Peter J. C. Moses, Apr 01 2016 *)