A133640 List of pairs n,4n, where n is the least unused number so far.

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

Offset: 1

Author

Jonathan Vos Post, Dec 28 2007

Keywords

• easy
• nonn

Comments

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

Examples

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

Crossrefs

Cf. A036552, A092401.

Programs

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 *)

Extensions

Data corrected by Giovanni Resta, Jun 19 2016