OFFSET
1,3
EXAMPLE
The nonempty subsets S of {1, 2, 3, 4} that contain only primes are these: {2}, {3}, {2,3}, thus, a(4) = 3.
MATHEMATICA
Table[-1 + 2^PrimePi[n], {n, 1, 70}]
PROG
(Python)
from sympy import primepi
def a(n): return 2**primepi(n) - 1
print([a(n) for n in range(1, 51)]) # Michael S. Branicky, Sep 24 2022
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Sep 24 2022
STATUS
approved