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A168043 Let S(1)={1} and, for n>1 let S(n) be the smallest set containing x+1, x+2, and 2*x for each element x in S(n-1). a(n) is the number of elements in S(n). 2
1, 2, 4, 7, 13, 23, 40, 68, 114, 189, 311, 509, 830, 1350, 2192, 3555, 5761, 9331, 15108, 24456, 39582, 64057 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..22.

FORMULA

It appears that a(n) = a(n-1) + a(n-2) + n - 3, for n>3.

From R. J. Mathar, Nov 18 2009: (Start)

Apparently: a(n) = 3*a(n-1) - 2*a(n-2) - a(n-3) + a(n-4) for n>5;

and a(n) = A000032(n+1) - n for n>1. (End)

From Ilya Gutkovskiy, Jul 07 2016: (Start)

It appears that the g.f. is x*(1 - x + x^4)/((1 - x)^2*(1 - x - x^2)); and the e.g.f. is phi*exp(phi*x) - exp(-x/phi)/phi - x*(1 + exp(x)) - 1, where phi is the golden ratio. (End)

It would be nice to have a proof for any one of these formulas. The others would then presumably follow easily. - N. J. A. Sloane, Jul 11 2016

EXAMPLE

Under the indicated set mapping we have {1} -> {2,3} -> {3,4,5,6} -> {4,5,6,7,8,10,12},..., so a(2)=2, a(3)=4, a(4)=7, etc.

CROSSREFS

Cf. A000032, A122554.

Sequence in context: A130709 A051013 A128609 * A114832 A239553 A319255

Adjacent sequences:  A168040 A168041 A168042 * A168044 A168045 A168046

KEYWORD

nonn,more

AUTHOR

John W. Layman, Nov 17 2009

EXTENSIONS

6 more terms from R. J. Mathar, Nov 18 2009

STATUS

approved

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Last modified March 19 13:29 EDT 2019. Contains 321330 sequences. (Running on oeis4.)