A080902 a(1)=1, a(n)=a(n-1)+2 if (n and n+3 are in the sequence), a(n)=a(n-1)+3 otherwise.

1, 4, 7, 9, 12, 15, 18, 21, 23, 26, 29, 31, 34, 37, 39, 42, 45, 47, 50, 53, 56, 59, 61, 64, 67, 69, 72, 75, 78, 81, 83, 86, 89, 91, 94, 97, 100, 103, 105, 108, 111, 113, 116, 119, 122, 125, 127, 130, 133, 135, 138, 141, 143, 146, 149, 151, 154, 157

Offset: 1

Author

Benoit Cloitre, Apr 01 2003

Keywords

• nonn

Examples

			a(9)=a(8)+2 since 9 and 12 are in the sequence.
		

Links

• Ivan Neretin, Table of n, a(n) for n = 1..1000

Programs

Mathematica

Fold[Append[#1, #1[[-1]] + If[MemberQ[#1, #2] && MemberQ[#1, #2 + 3], 2, 3]] &, {1}, Range[2, 58]] (* Ivan Neretin, Mar 03 2016 *)

Formula

Conjectured to be asymptotic to r*n, where r=2.72... Is r the largest root of q^2-3*q+3/4, which is 2.72474487139...?

Extensions

Corrected by T. D. Noe, Nov 02 2006