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A327475
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Number of subsets of {1..n} whose mean is an integer, where {} has mean 0.
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38
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1, 2, 3, 6, 9, 16, 27, 46, 77, 136, 239, 426, 769, 1400, 2571, 4762, 8857, 16568, 31139, 58734, 111165, 211044, 401695, 766418, 1465489, 2807672, 5388783, 10359850, 19946833, 38459624, 74251095, 143524762, 277742489, 538043664, 1043333935, 2025040766, 3933915349
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OFFSET
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0,2
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LINKS
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FORMULA
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EXAMPLE
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The a(0) = 1 through a(5) = 16 subsets:
{} {} {} {} {} {}
{1} {1} {1} {1} {1}
{2} {2} {2} {2}
{3} {3} {3}
{1,3} {4} {4}
{1,2,3} {1,3} {5}
{2,4} {1,3}
{1,2,3} {1,5}
{2,3,4} {2,4}
{3,5}
{1,2,3}
{1,3,5}
{2,3,4}
{3,4,5}
{1,2,4,5}
{1,2,3,4,5}
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MAPLE
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with(numtheory):
b:= n-> add(2^(n/d)*phi(d), d=select(x-> x::odd, divisors(n)))/n:
a:= proc(n) option remember; `if`(n=0, 1, b(n)-1+a(n-1)) end:
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MATHEMATICA
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Table[Length[Select[Subsets[Range[n]], #=={}||IntegerQ[Mean[#]]&]], {n, 0, 10}]
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PROG
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(Python)
from sympy import totient, divisors
def A327475(n): return sum((sum(totient(d)<<k//d-1 for d in divisors(k>>(~k&k-1).bit_length(), generator=True))<<1)//k for k in range(1, n+1))-n+1 # Chai Wah Wu, Feb 22 2023
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CROSSREFS
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If the subset is required to contain n, we get A063776.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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