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A260107 Lexicographically first increasing sequence of positive integers such that there are exactly a(k) terms less than or equal to 3*a(k), for each k. 3
1, 4, 5, 6, 13, 16, 19, 20, 21, 22, 23, 24, 25, 40, 41, 42, 49, 50, 51, 58, 61, 64, 67, 70, 73, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 121, 124, 127, 128, 129, 130, 131, 132, 133, 148, 151, 154, 155, 156, 157, 158, 159, 160, 175, 176 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

See A130011 and A260139 for other variants of this self-describing sequence.

LINKS

Nathaniel Johnston, Table of n, a(n) for n = 1..10000

FORMULA

a(n) <= 3n-2, and there are infinitely many indices (namely, all those of the form n = a(k)+1 for some k) for which equality holds.

MAPLE

l:=[1, 4]:for n from 2 to 20 do for j from l[n-1]+1 to `if`(n=2, l[n]-1, l[n]) do l:=[op(l), max(3*l[n-1], op(l))+1]: od: od: l; # Nathaniel Johnston, Apr 27 2011

PROG

(PARI) a=vector(100, i, 1); i=v=1; for(k=2, #a, if(k>a[i], v=3*a[i]; i++); a[k]=v++)

CROSSREFS

Cf. A130011, A037988, A094591.

Sequence in context: A055033 A089119 A063833 * A167434 A139061 A029645

Adjacent sequences:  A260104 A260105 A260106 * A260108 A260109 A260110

KEYWORD

nonn,easy

AUTHOR

M. F. Hasler, Jul 16 2015

STATUS

approved

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Last modified April 6 11:51 EDT 2020. Contains 333273 sequences. (Running on oeis4.)