This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A357215 #10 Sep 26 2022 20:10:37
%S A357215 0,1,3,3,7,7,15,15,15,15,31,31,63,63,63,63,127,127,255,255,255,255,
%T A357215 511,511,511,511,511,511,1023,1023,2047,2047,2047,2047,2047,2047,4095,
%U A357215 4095,4095,4095,8191,8191,16383,16383,16383,16383,32767,32767,32767,32767
%N A357215 a(n) = number of nonempty subsets S of {1, 2, ..., n} that contain only primes.
%F A357215 a(n) = -1 + A048656(n).
%F A357215 a(n) = 2^A000720(n) - 1.
%e A357215 The nonempty subsets S of {1, 2, 3, 4} that contain only primes are these: {2}, {3}, {2,3}, thus, a(4) = 3.
%t A357215 Table[-1 + 2^PrimePi[n], {n, 1, 70}]
%o A357215 (Python)
%o A357215 from sympy import primepi
%o A357215 def a(n): return 2**primepi(n) - 1
%o A357215 print([a(n) for n in range(1, 51)]) # _Michael S. Branicky_, Sep 24 2022
%Y A357215 Cf. A000720, A048656, A089819, A357214.
%K A357215 nonn,easy
%O A357215 1,3
%A A357215 _Clark Kimberling_, Sep 24 2022