OFFSET
1,2
COMMENTS
a(n) is the number of sign mappings X:([n] choose 2) -> {+,-} such that for any ordered 3-tuple a<b<c we have X(ab)X(ac)X(bc) not equal to -++.
PROG
(PARI)
a(n)={
local(M=Map(Mat([1, 1])));
my(acc(p, v)=my(z); mapput(M, p, if(mapisdefined(M, p, &z), z+v, v)));
my(proc(p, m)=for(k=0, poldegree(p), acc(p + x*(1 + x)^k, polcoef(p, k)*m)));
for(r=1, n, my(src=Mat(M)); M=Map(); for(i=1, matsize(src)[1], proc(src[i, 1], src[i, 2])));
vecsum(Mat(M)[, 2])
} \\ Andrew Howroyd, Jan 06 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Manfred Scheucher and Robert Lauff, Jan 05 2024
EXTENSIONS
Terms a(12) and beyond from Andrew Howroyd, Jan 06 2024
STATUS
approved