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.
f[n_] := Plus @@ MoebiusMu[ Drop[ Divisors[n], 1] - 1]; t = Table[0, {100}]; Do[ a = f[n]; If[Positive[a] && a < 101 && t[[a]] == 0, t[[a]] = n], {n, }]; t
f[n_] := Plus @@ MoebiusMu[ Drop[ Divisors[n], 1] - 1]; s = Table[0, {100}]; Do[ a = f[n]; If[ !Positive[a] && a > -101 && s[[ -a]] == 0, s[[ -a]] = n], {n, }]; s
f[n_] := Plus @@ MoebiusMu[ Drop[ Divisors[n], 1] - 1]; Select[ Range[ 192], f[ # ] == 0 &]
The divisors of 12 are 1, 2, 3, 4, 6 and 12. So a(12) = mu(2) + mu(3) + mu(4) + mu(5) + mu(7) + mu(13) = -1 - 1 + 0 - 1 - 1 - 1 = -5.
a[n_] := Plus @@ MoebiusMu[Divisors[n] + 1]; Table[ a[n], {n, 105}] (* Robert G. Wilson v, Nov 01 2004 *)
a(n) = sumdiv(n, d, moebius(d+1)); \\ Amiram Eldar, Jun 06 2025
Table[Sum[MoebiusMu[Floor[n/k]], {k, 1, n}], {n, 1, 85}]
Table[1 - Sum[DivisorSum[k, MoebiusMu[# - 1] &, # > 1 &], {k, 1, n}], {n, 1, 85}]
nmax = 85; CoefficientList[Series[(1/(1 - x)) (x - Sum[MoebiusMu[k - 1] x^k/(1 - x^k), {k, 2, nmax}]), {x, 0, nmax}], x] // Rest
a(n) = sum(k=1, n, moebius(n\k)); \\ Michel Marcus, Feb 14 2020
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