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%I #6 Jun 19 2016 14:58:24
%S 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,
%T 17,68,18,72,19,76,21,84,22,88,23,92,25,100,26,104,27,108,29,116,30,
%U 120,31,124,32,128,33,132,34,136,35,140,37,148,38,152,39,156
%N List of pairs n,4n, where n is the least unused number so far.
%C A permutation of the natural numbers. This is to 4 as A036552 is to 2 and as A092401.
%e 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:
%e ===========================================================================
%e n...|.1..2..3..4..5..6..7..8..9..10..11..12..13..14..15..16..17..18..19..20
%e ===========================================================================
%e k=2.|.1..2..3..6..4..8..5.10..7..14...9..18..11..22..12..24..13..26..15..30
%e k=3.|.1..3..2..6..4.12..5.15..7..21...8..24...9..27..10..30..11..33..13..39
%e k=4.|.1..4..2..8..3.12..5.20..6..24...7..28...9..36..10..40..11..44..13..52
%e k=5.|.1..5..2.10..3.15..4.20..6..30...7..35...8..40...9..45..11..55..12..60
%e k=6.|.1..6..2.12..3.18..4.24..5..30...7..42...8..48...9..54..10..60..11..66
%e k=7.|.1..7..2.14..3.21..4.28..5..35...6..42...8..56...9..63..10..70..11..77
%e k=8.|.1..8..2.16..3.24..4.32..5..40...6..48...7..56...9..72..10..80..11..88
%e k=9.|.1..9..2.18..3.27..4.36..5..45...6..54...7..63...8..72..10..90..11..99
%e k=10|.1.10..2.20..3.30..4.40..5..50...6..60...7..70...8..80...9..90..11.110
%e ===========================================================================
%t 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 *)
%Y Cf. A036552, A092401.
%K easy,nonn
%O 1,2
%A _Jonathan Vos Post_, Dec 28 2007
%E Data corrected by _Giovanni Resta_, Jun 19 2016
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