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A332072
The Blue Comma sequence: Differences of indices of even terms gives back the sequence; lexicographically earliest such permutation of positive integers.
0
1, 2, 4, 3, 6, 5, 7, 9, 8, 11, 13, 10, 15, 17, 19, 21, 23, 12, 25, 27, 29, 31, 14, 33, 35, 37, 39, 41, 43, 16, 45, 47, 49, 51, 53, 55, 57, 59, 18, 61, 63, 65, 67, 69, 71, 73, 20, 75, 77, 79, 81, 83, 85, 87, 89, 91, 93, 22, 95, 97, 99, 101, 103, 105, 107, 109, 111, 113, 115, 117, 24, 119, 121, 123, 125, 127
OFFSET
1,2
LINKS
Eric Angelini, Yellow commas, pink, red, blue..., personal blog "Cinquante signes" on blogspot.com, May 17 2020
EXAMPLE
The even terms are at indices 2, 3, 5, 9, 12, 18, 23, 30, ...; first differences give 1, 2, 4, 3, 6, 5, 7, ... = the sequence itself.
PROG
(PARI) upto(N)={my(a=Vec([1, 2], N), i=1, L=2); local(U=[2], nxt(e)=while(#U>1&&U[2]==U[1]+1, U=U[^1]); my(t=U[1]); until( t++%2!=e && !setsearch(U, t), ); U=setunion(U, [t]); t ); for(n=3, N, a[n]=nxt(n-L==a[i] && i++ && L=n)); a}
CROSSREFS
Cf. A197756 (the Yellow Comma sequence: count digits between odd terms).
Sequence in context: A255552 A255551 A366949 * A330340 A328696 A053629
KEYWORD
nonn
AUTHOR
Eric Angelini and M. F. Hasler, May 19 2020
STATUS
approved