OFFSET
4,2
EXAMPLE
a(5) = 2 since when U(5) = {1,2,3,4} acts naturally on the five 4-subsets {0,1,2,3}, {0,1,2,4}, {0,1,3,4}, {0,2,3,4}, {1,2,3,4} of Z/(5) the orbits are {{0,1,2,3},{0,1,2,4}, {0,1,3,4}, {0,2,3,4}} and {{1,2,3,4}}.
MAPLE
g:= proc(n) local U, S, C, R, u, s, Cr, us, v;
U:= select(t -> igcd(n, t)=1, [$1..n-1]);
S:= combinat:-choose({$0..n-1}, 4);
C:= S;
for s in S do R[s]:= s od;
for u in U do
Cr:= NULL;
for s in C do
us:= map(t -> u*t mod n, s);
v:= R[us];
while R[v] <> v do v:= R[v] od;
if v <> s then R[s]:= v; Cr:= Cr, s fi
od;
C:= C minus {Cr};
od;
nops(C)
end proc;
map(g, [$4..60]); # Robert Israel, Nov 28 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
W. Edwin Clark, Jul 15 2001
EXTENSIONS
Offset corrected, and more terms by Robert Israel, Nov 28 2022
STATUS
approved