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 A064355 #18 Jun 05 2025 00:43:27
%S A064355 2,2,2,4,6,10,18,32,56,102,186,340,630,1170,2182,4096,7710,14560,
%T A064355 27594,52428,99858,190650,364722,699040,1342176,2581110,4971008,
%U A064355 9586980,18512790,35791358,69273666,134217728,260300986,505290270,981706806,1908874240,3714566310
%N A064355 Number of subsets of {1,2,..n} that sum to 1 mod n.
%H A064355 <a href="/index/Su#subsetsums">Index entries for sequences related to subset sums modulo m</a>.
%F A064355 a(n) = 2*A000048(n).
%F A064355 a(n) = (1/n) * sum_{d divides n and d is odd} 2^(n/d) * mu(d); (mu(d) is the Moebius function, sequence A008683).
%e A064355 a(7) = 18 because there are 18 subsets of {1,2,3,4,5,6,7} which sum to 1 mod 7:{1}, {1,7}, {2,6}, {3,5}, {1,2,5}, {1,3,4}, {2,6,7}, {3,5,7}, {4,5,6}, {1,2,5,7}, {1,3,4,7}, {1,3,5,6}, {2,3,4,6}, {4,5,6,7}, {1,2,3,4,5}, {1,3,5,6,7}, {2,3,4,6,7}, {1,2,3,4,5,7}.
%t A064355 a[n_] := Block[{d = Select[Divisors@n, OddQ@ # &]}, Plus @@ (2^(n/d)*MoebiusMu@d)/n]; Array[a, 35] (* _Robert G. Wilson v_, Feb 20 2006 *)
%o A064355 (PARI) a(n) = sumdiv(n, d, (d % 2) * 2^(n/d) * moebius(d)) / n; \\ _Amiram Eldar_, Jun 05 2025
%Y A064355 Cf. A000048, A008683, A063776.
%K A064355 nonn
%O A064355 1,1
%A A064355 Dan Fux (dan.fux(AT)OpenGaia.com or danfux(AT)OpenGaia.com), Sep 25 2001
%E A064355 More terms from _Vladeta Jovovic_, Sep 27 2001