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A133640 List of pairs n,4n, where n is the least unused number so far. 1
1, 4, 2, 8, 3, 12, 5, 20, 6, 24, 7, 28, 9, 36, 10, 40, 11, 44, 13, 52, 14, 56, 15, 60, 16, 64, 17, 68, 18, 72, 19, 76, 21, 84, 22, 88, 23, 92, 25, 100, 26, 104, 27, 108, 29, 116, 30, 120, 31, 124, 32, 128, 33, 132, 34, 136, 35, 140, 37, 148, 38, 152, 39, 156 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

A permutation of the natural numbers. This is to 4 as A036552 is to 2 and as A092401.

LINKS

Table of n, a(n) for n=1..64.

EXAMPLE

Equivalently, this is row 4 of the array A[k,n] = n-th value of the sequence: list of pairs n,k*n, where n is the least unused number so far. That array begins:

===========================================================================

n...|.1..2..3..4..5..6..7..8..9..10..11..12..13..14..15..16..17..18..19..20

===========================================================================

k=2.|.1..2..3..6..4..8..5.10..7..14...9..18..11..22..12..24..13..26..15..30

k=3.|.1..3..2..6..4.12..5.15..7..21...8..24...9..27..10..30..11..33..13..39

k=4.|.1..4..2..8..3.12..5.20..6..24...7..28...9..36..10..40..11..44..13..52

k=5.|.1..5..2.10..3.15..4.20..6..30...7..35...8..40...9..45..11..55..12..60

k=6.|.1..6..2.12..3.18..4.24..5..30...7..42...8..48...9..54..10..60..11..66

k=7.|.1..7..2.14..3.21..4.28..5..35...6..42...8..56...9..63..10..70..11..77

k=8.|.1..8..2.16..3.24..4.32..5..40...6..48...7..56...9..72..10..80..11..88

k=9.|.1..9..2.18..3.27..4.36..5..45...6..54...7..63...8..72..10..90..11..99

k=10|.1.10..2.20..3.30..4.40..5..50...6..60...7..70...8..80...9..90..11.110

===========================================================================

MATHEMATICA

L = {1, 4}; Do[x=First[Complement[Range[Max[L] + 1], L]]; L = Join[L, {x, 4*x}], {38}]; L (* Giovanni Resta, Jun 19 2016 *)

CROSSREFS

Cf. A036552, A092401.

Sequence in context: A026185 A026209 A198473 * A065171 A026192 A026142

Adjacent sequences:  A133637 A133638 A133639 * A133641 A133642 A133643

KEYWORD

easy,nonn

AUTHOR

Jonathan Vos Post, Dec 28 2007

EXTENSIONS

Data corrected by Giovanni Resta, Jun 19 2016

STATUS

approved

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Last modified February 28 00:28 EST 2020. Contains 332319 sequences. (Running on oeis4.)