OFFSET
1,2
EXAMPLE
a(1) = 1 via [1];
a(2) = 2 via [1,2];
a(3) = 6 via [1,2,3];
a(4) = 6 via [1,2,3,1];
a(5) = 24 via [1,2,3,1,4];
a(6) = 48 via [1,2,3,1,4,2];
a(7) = 120 via [1,2,3,1,4,5,1];
a(8) = 240 via [1,2,3,1,4,2,5,1];
a(9) = 1440 via [1,2,3,1,4,2,5,1,6];
a(10) = 2880 via [1,2,3,1,4,2,5,1,6,2];
a(11) = 10080 via [1,2,3,1,4,2,5,1,6,7,1].
Examples:
[1,1] is invalid because 1 is self-adjacent.
[1,2,3,1,2] is invalid because the substring [1,2] appears twice.
[1,2,1] is invalid because the substring [1,2] appears twice (once forward and once backward).
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Peter Kagey, Feb 07 2017
STATUS
approved