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A275583
The multiples of 3 of the sequence appear in blocks. The successive sizes of these blocks are given by the sequence itself.
0
1, 2, 3, 4, 5, 6, 9, 7, 8, 10, 11, 12, 15, 18, 13, 14, 16, 17, 19, 20, 21, 24, 27, 30, 22, 23, 25, 26, 28, 29, 31, 32, 33, 36, 39, 42, 45, 34, 35, 37, 38, 40, 41, 43, 44, 46, 47, 48, 51, 54, 57, 60, 63, 49, 50, 52, 53, 55, 56, 58, 59, 61, 62, 64, 65, 66, 69, 72, 75, 78, 81, 84, 87, 90, 67, 68, 70, 71, 73, 74, 76, 77, 79, 80, 82, 83, 85, 86, 88, 89, 91, 92, 93
OFFSET
1,2
COMMENTS
The sequence is started with a(1)=1 and always extended with the smallest integer not yet used that doesn't lead to a contradiction.
The sequence is a permutation of the natural numbers.
EXAMPLE
The blocks of multiples of 3 are indicated here by parentheses; the successive block-sizes are 1, 2, 3, 4, 5, ... which reproduces the sequence itself:
1,2,(3),4,5,(6,9),7,8,10,11,(12,15,18),13,14,16,17,19,20,(21,24,27,30),22,23,25,26,28,29,31,32,(33,36,39,42,45),34,...
MATHEMATICA
a[1]=1; a[n_]:=a[n]=Block[{k=1}, While[MemberQ[s=Array[a, n-1], k]||(g=Length/@Select[SplitBy[Join[s, {k}], Mod[#, 3]==0&], Mod[First@#, 3]==0&]; g!=s[[;; Length@g]]), If[Mod[k, 3]==0&&FreeQ[s, k], Break[], k++]]; k]; Array[a, 93] (* Giorgos Kalogeropoulos, May 12 2022 *)
CROSSREFS
Sequence in context: A345900 A029964 A368240 * A295088 A332990 A257815
KEYWORD
nonn,base
AUTHOR
Eric Angelini, Aug 02 2016
STATUS
approved