OFFSET
1,2
EXAMPLE
Let a(n) be this sequence and b(n)=|a(n)-a(n+1)| be the inverse permutation of this sequence.
After a(1)=1, a(2)=2, a(3)=4, the next term, a(4), cannot be a repeat of 1,2, or 4 since by definition a(n) must be a permutation of the positive integers.
It cannot be 3,5, or 6, as that would force b(3)=1 or 2 (a repeat of b(1)=1, or b(2)=2).
We cannot have a(4)=7, because b(3)=3 implies a(3)=3, which contradicts a(3)=4.
We cannot have a(4)=8, because b(3)=4 implies a(4)=3.
We cannot have a(4)=9, because b(3)=5 implies a(5)=3, and b(4)=|a(5)-a(4)|=6 which contradicts b(4)=3 as implied by a(3)=4.
Therefore a(4)=10 is the smallest value of a(4) which will not generate a contradiction.
CROSSREFS
KEYWORD
nice,nonn
AUTHOR
Andrew Weimholt, Sep 04 2010
STATUS
approved