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A257470 a(n+1) = least number not occurring earlier such that max{a(n), a(n+1)} >= 2 min{a(n), a(n+1)}; a(0) = 0. 2
0, 1, 2, 4, 8, 3, 6, 12, 5, 10, 20, 7, 14, 28, 9, 18, 36, 11, 22, 44, 13, 26, 52, 15, 30, 60, 16, 32, 64, 17, 34, 68, 19, 38, 76, 21, 42, 84, 23, 46, 92, 24, 48, 96, 25, 50, 100, 27, 54, 108, 29, 58, 116, 31, 62, 124, 33, 66, 132, 35, 70, 140, 37, 74, 148, 39, 78, 156, 40, 80, 160, 41, 82, 164, 43, 86, 172, 45, 90, 180, 47, 94, 188, 49, 98, 196, 51 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Otherwise said, a(n+1) is either the smallest number not occurring earlier if this number is smaller than a(n)/2, or else a(n+1) is the least unused number >= 2 a(n).

This is a simpler variant of A139080. Here the well-definedness of the infinite sequence is guaranteed.

This is a permutation of the nonnegative integers. Indeed, any number m appears either "early" as successor of m/2, or "late" as successor a(n+1) of some a(n) >= 2m (in which case n+1 = 3k-1, see below).

After the two initial terms, the following 3-periodic pattern repeats: a(3k-1) = the smallest term not occurring earlier, a(3k) = 2 a(3k-1), and a(3k+1) = 2 a(3k), since at this point the smallest unused number is necessarily > a(3k)/2 = a(3k-1), and therefore, using that number, the maximum would be smaller than twice the minimum.

In spite of the simplicity of the pattern, it seems not easy to give an explicit formula of a(n), i.e., for a(3k-1), cf. formulas. I conjecture the following properties:

(i) For all k>0, a(3k+2) = a(3k-1)+2 except for k in S = {1, 8, 9, 13, 14, 22, 23, 31, 32, 40, 41, 49, 50,...} where a(3k+2) = a(3k-1)+1.

(ii) Let D = (7, 1, 4, 1, 8, 1, 8, 1, 8, 1, 8, 1, 8, ...) be the first differences of the sequence S. Then D(2k) = 1 for all k>0.

(iii) D(2k+1) = 4 or 8 for all k>0; except for the first one, the 4's always come twice in a row (e.g., for k in {8, 9}, {13, 14}, {22, 23}, ...).

(iv) The length of runs of 8's in the subsequence D(2k+1) are: 6 (k=2..7), 3 (k=10..12), 7 (k=15..21), 7 (k=24..30), 7 (k=33..39), 7 (k=42..48), 7 (k=51..57), 7 (k=60..66), 3 (k=69..71), 3 (k=74..76), 7,... It seems that except for the first two terms, this consists of runs of 7 of various length, but always interrupted by two 3's.

LINKS

Table of n, a(n) for n=0..86.

OEIS Index entries of sequences related to permutations of the nonnegative integers.

FORMULA

For all k>0, a(3k-1) = least number not occurring earlier; a(3k) = 2 a(3k-1); a(3k+1) = 2 a(3k).

EXAMPLE

After a(0)=0, a(1)=1 is the least unused number such that max{1,0} >= 2 min{1,0} = 0.

After a(2)=2, a(3)=4 is the least unused number such that max{2,4} >= 2 min{2,4} = 4.

After a(3)=4, a(4)=8 is the least unused number such that max{8,4} >= 2 min{8,4} = 8, because the only smaller unused number, 3, would not satisfy the requirement.

After a(4)=8, a(5)=3 is the least unused number such that max{8,3} >= 2 min{8,3} = 6.

MATHEMATICA

f[n_] := Block[{s = {1}}, For[i = 2, i <= n, i++, k = 1; While[Nand[! MemberQ[s, k], Max[k, s[[i - 1]]] >= 2 Min[k, s[[i - 1]]]], k++]; AppendTo[s, k]]; s]; f@ 86 (* Michael De Vlieger, Apr 25 2015 *)

PROG

(PARI) {a=vector(2000); u=[]; a[1]=1; for(n=2, #a, u=setunion(u, [a[n-1]]); while(#u>1&&u[2]==u[1]+1, u=u[2..-1]); if( u[1]*2+1 < a[n-1], a[n]=u[1]+1, a[n]=a[n-1]*2; while(setsearch(u, a[n]), a[n]++)))}

CROSSREFS

Sequence in context: A110217 A243062 A232645 * A139080 A036118 A247555

Adjacent sequences:  A257467 A257468 A257469 * A257471 A257472 A257473

KEYWORD

nonn

AUTHOR

M. F. Hasler, Apr 25 2015

STATUS

approved

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Last modified January 22 09:52 EST 2019. Contains 319363 sequences. (Running on oeis4.)