OFFSET
1,2
COMMENTS
Also the number of sign mappings X:([n] choose 2) -> {+,-} such that for any ordered 3-tuple abc we have X(ab)X(ac)X(bc) not in {++-,+--}.
FORMULA
a(n) = 1 + Sum_{k=1..n-1} (2^k-1)*2^((n-1-k)*k).
MAPLE
seq(1 + add((2^k-1)*2^((n-1-k)*k), k=1..n-1), n=1..20); # Georg Fischer_, May 28 2024
PROG
(Python) def f(n): return 1+sum((2**k-1)*2**((n-1-k)*k) for k in range(1, n))
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert Lauff and Manfred Scheucher, Jan 05 2024
EXTENSIONS
a(20), a(21) joined by Georg Fischer, May 28 2024
STATUS
approved