OFFSET
0,3
COMMENTS
Note for n<5 there are no such subsequences, so those values are trivially n!. Also note it is possible for a permutation to have both i,i+2,i+4 and i,i-2,i-4 triples, as in an example from n=7: (2,4,6,5,3,1,0). This permutation is not counted by a(7).
EXAMPLE
For n=5 there are 5!-a(5)=12 permutations with i,i+2,i+4 or i,i-2,i-4 triples. An examples of one is (4,2,0,1,3).
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Isaac Lambert, Mar 10 2010
EXTENSIONS
a(0)-a(4) and a(10)-a(19) from Alois P. Heinz, Apr 14 2021
STATUS
approved