OFFSET
1,3
FORMULA
a(p) = a(p-1) + 1 for prime p > 2. - Michael S. Branicky, Sep 30 2022
EXAMPLE
a(11) = 10 subsets: {1}, {3}, {5}, {7}, {9}, {11}, {2, 6}, {2, 3, 6}, {3, 6, 10} and {3, 5, 6, 10}.
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
@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