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A007620
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Numbers m such that every k <= m is a sum of proper divisors of m (for m>1).
(Formerly M4095)
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5
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1, 6, 12, 18, 20, 24, 28, 30, 36, 40, 42, 48, 54, 56, 60, 66, 72, 78, 80, 84, 88, 90, 96, 100, 104, 108, 112, 120, 126, 132, 140, 144, 150, 156, 160, 162, 168, 176, 180, 192, 196, 198, 200, 204, 208, 210, 216, 220, 224, 228, 234, 240, 252, 260, 264, 270, 272, 276, 280, 288, 294, 300, 304, 306
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OFFSET
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1,2
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COMMENTS
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This sequence was formerly called "practical numbers (second definition)" because it was thought this was the definition used in Srinivasan's original paper. However, in Srinivasan's paper, one can read that his definition is "k < m". Stewart proves that Srinivasan's definition is equivalent to requiring every k <= sigma(m) be the sum of distinct divisors of m. This sequence is a subsequence of the practical numbers, A005153. - T. D. Noe, Apr 02 2010
A005153 without terms larger than 1 that are almost-perfect numbers (numbers k such that sigma(k) = 2*k-1, the only known such numbers are the powers of 2, A000079). - Amiram Eldar, Apr 07 2023
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REFERENCES
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Ross Honsberger, Mathematical Gems, M.A.A., 1973, p. 113.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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MATHEMATICA
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DeleteCases[ A005835, q_/; (Count[ CoefficientList[ Series[ Times@@( (1+z^#)& /@ Divisors[ q ] ), {z, 0, q} ], z ], 0 ]>0) ] (* Wouter Meeussen *)
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PROG
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(Haskell)
a007620 n = a007620_list !! (n-1)
a007620_list = 1 : filter (\x -> all (p $ a027751_row x) [1..x]) [2..]
where p _ 0 = True
p [] _ = False
p ds'@(d:ds) m = d <= m && (p ds (m - d) || p ds m)
(Python)
from itertools import count, islice
from sympy import divisors
def A007620_gen(startvalue=1): # generator of terms >= startvalue
for m in count(max(startvalue, 1)):
if m == 1:
yield 1
else:
c = {0}
for d in divisors(m, generator=True):
if d < m:
c |= {a+d for a in c}
if all(a in c for a in range(m+1)):
yield m
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CROSSREFS
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KEYWORD
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nonn,nice,easy
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AUTHOR
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STATUS
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approved
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