OFFSET
1,2
COMMENTS
Examples:
[1,1] is not a valid sequence because 1 is self-adjacent.
[1,2,3,1,2] is not valid because the substring [1,2] appears twice.
EXAMPLE
a(1) = 1 via [1];
a(2) = 2 via [1,2];
a(3) = 2 via [1,2,1];
a(4) = 6 via [1,2,1,3];
a(5) = 6 via [1,2,1,3,1];
a(6) = 24 via [1,2,1,3,1,4];
a(7) = 24 via [1,2,1,3,1,4,1];
a(8) = 120 via [1,2,1,3,1,4,1,5];
a(9) = 120 via [1,2,1,3,1,4,1,5,1];
a(10) = 576 via [1,2,1,3,1,4,2,3,4,1];
a(11) = 720 via [1,2,1,3,1,4,1,5,1,6,1].
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Peter Kagey, Feb 07 2017
STATUS
approved