OFFSET
1,2
MATHEMATICA
sw[n_] := n!/(Floor[n/2]!)^2; lim = 40; For[p = 0; a = f = Table[sw[n], {n, lim}], p =!= a, p = a; a = Select[Union@@Outer[Times, f, a], #<= sw[lim]&]]; a (* Hans Havermann, Sep 09 2014 *)
PROG
(Sage)
# For example prod_hull(A008578) are the natural numbers.
def prod_hull(f, K):
S = []; newS = []
n = 0
while f(n) <= K:
newS.append(f(n))
n += 1
while newS != S:
S = newS; T = []
for s in S:
M = map(lambda n: n*s , S)
T.extend(filter(lambda n: n <= K, M))
newS = Set(T).union(Set(S))
return sorted(newS)
prod_hull(lambda n: factorial(n)/factorial(n//2)^2, 1024)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Peter Luschny, Sep 09 2014
STATUS
approved