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A175522
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A000120-perfect numbers.
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27
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2, 25, 95, 111, 119, 123, 125, 169, 187, 219, 221, 247, 289, 335, 365, 411, 415, 445, 485, 493, 505, 629, 655, 685, 695, 697, 731, 767, 815, 841, 871, 943, 949, 965, 985, 1003, 1139, 1207, 1241, 1261, 1263, 1273, 1343, 1387, 1465, 1469, 1507, 1513, 1529, 1563
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OFFSET
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1,1
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COMMENTS
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Let A(n), n>=1, be an infinite positive sequence.
We call a number n:
A-deficient if Sum{d|n, d<n} A(d) < A(n),
A-abundant if Sum{d|n, d<n} A(d) > A(n),
and
A-perfect if Sum{d|n, d<n} A(d) = A(n),
depending on the sum over the proper divisors of n.
The definition generalizes the standard nomenclature of deficient (A005100), abundant (A005101) and perfect numbers (A000396), which is recovered by setting A(n) = n = A000027(n).
Conjecture: if there exist infinitely many A-deficient numbers and infinitely many A-abundant numbers, then there exist infinitely many A-perfect numbers.
Note that the sequence contains squares of all Fermat primes larger than 3 (see A019434). [This would also hold for squares of any hypothetical Fermat primes after the fifth one, 65537. Comment clarified by Antti Karttunen, May 14 2015]
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LINKS
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EXAMPLE
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MATHEMATICA
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binw[n_] := DigitCount[n, 2, 1]; Select[Range[1500], binw[#] == DivisorSum[#, binw[#1] &]/2 &] (* Amiram Eldar, Dec 14 2020 *)
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PROG
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(Sage) A000120 = lambda x: x.digits(base=2).count(1)
(Haskell)
import Data.List (elemIndices)
a175522 n = a175522_list !! (n-1)
a175522_list = map (+ 1) $ elemIndices 0 a192895_list
(Python)
from sympy import divisors
def A000120(n): return bin(n).count('1')
def aupto(limit):
alst = []
for m in range(1, limit+1):
if A000120(m) == sum(A000120(d) for d in divisors(m)[:-1]): alst += [m]
return alst
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CROSSREFS
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Subsequence of A257691 (non-abundant version).
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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