1,2

Sequence suggested by Leroy Quet on the SeqFan mailing list.

Table of n, a(n) for n=1..15.

a(4)=8, since the 8 permutations (1,2,3,4), (1,4,3,2), (2,1,4,3), (2,3,4,1), (4,1,2,3), (3,2,1,4), (3,4,1,2), (4,3,2,1) satisfy the property.

Cf. A076220, A117541.

Sequence in context: A019199 A094678 A076507 * A045653 A235320 A152158

Adjacent sequences: A117539 A117540 A117541 * A117543 A117544 A117545

nonn

Giovanni Resta, Mar 28 2006

approved