OFFSET
1,5
COMMENTS
Let x_0 be a permutation on {1, 2, 3, ..., n}. Let x_k(i) be a function defined when 0 < i <= n - k that is constructed as follows:
If x_k(i + 1) >= x_k(i), then x_{k+1}(i) = x_k(i + 1) - x_k(i).
Otherwise, x_{k+1}(i) = x_k(i + 1) + x_k(i).
a(n) is the number of permutations x_0 that satisfy x_{n-1}(1) = 0.
Comments from Olivier Gérard, Jun 04 2024 (Start)
The sequence of number of different values is:
1, 2, 4, 9, 32, 75, 179, 230, 933
The sequence of maxima of this process is A001792:
1, 3, 8, 20, 48, 112, 256, 576, 1280
Indeed, the maxima is attained only once and always for the last permutation in lexicographic order : n, n-1, n-2, ..., 1 (End).
EXAMPLE
For n=5, one of the a(5) = 3 solutions is (1, 4, 5, 2, 3), whose trajectory to 0 is
1 4 5 2 3
3 1 7 1
4 6 8
2 2
0
PROG
(Python)
from itertools import permutations
def f(t):
if len(t) == 1: return t[0]
return f([t[i]+t[i+1] if t[i+1]<t[i] else t[i+1]-t[i] for i in range(len(t)-1)])
def a(n): return sum(1 for p in permutations(range(1, n+1)) if f(p) == 0)
print([a(n) for n in range(1, 10)]) # Michael S. Branicky, May 30 2024
CROSSREFS
KEYWORD
nonn,nice,hard,more
AUTHOR
Bryle Morga, May 30 2024
EXTENSIONS
a(12)-a(13) from Michael S. Branicky, May 30 2024
a(14)-a(16) from Bert Dobbelaere, Jun 09 2024
STATUS
approved