OFFSET
1,4
FORMULA
a(p) = a(p-1) for prime p > 2. - Michael S. Branicky, Sep 30 2022
EXAMPLE
a(11) = 9 subsets: {2}, {4}, {6}, {8}, {10}, {3, 6}, {1, 3, 6}, {3, 4, 6} and {1, 2, 3, 6}.
PROG
(Python)
from fractions import Fraction
from functools import lru_cache
def cond(s, c): h = c/s; return h.denominator == 1 and h.numerator&1 == 0
@lru_cache(maxsize=None)
def b(n, s, c):
if n == 0: return int (c > 0 and cond(s, c))
return b(n-1, s, c) + b(n-1, s+Fraction(1, n), c+1)
a = lambda n: b(n, 0, 0)
print([a(n) for n in range(1, 18)]) # Michael S. Branicky, Sep 29 2022
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Ilya Gutkovskiy, Sep 27 2022
EXTENSIONS
a(24)-a(35) from Michael S. Branicky, Sep 30 2022
STATUS
approved