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A005153
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Practical numbers: positive integers n such that every k <= sigma(n) is a sum of distinct divisors of n. Also called panarithmic numbers.
(Formerly M0991)
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55
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1, 2, 4, 6, 8, 12, 16, 18, 20, 24, 28, 30, 32, 36, 40, 42, 48, 54, 56, 60, 64, 66, 72, 78, 80, 84, 88, 90, 96, 100, 104, 108, 112, 120, 126, 128, 132, 140, 144, 150, 156, 160, 162, 168, 176, 180, 192, 196, 198, 200, 204, 208, 210, 216, 220, 224, 228, 234, 240, 252
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OFFSET
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1,2
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COMMENTS
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Equivalently, positive integers n such that every number k <= n is a sum of distinct divisors of n.
2^r is a member for all r as every number < = sigma(2^r) = 2^(r+1)-1 is a sum of a distinct subset of divisors {1,2,2^2,...2^n}. - Amarnath Murthy, Apr 23 2004
Also, numbers n such that A030057(n) > n. This is a consequence of the following theorem (due to Stewart), found at the McLeman link: An integer m >= 2 with factorization Product_{i=1}^k p_i^e_i with the p_i in ascending order is practical if and only if p_1 = 2 and, for 1 < i <= k, p_i <= sigma(Product_{j < i} p_j^e_j) + 1. - Franklin T. Adams-Watters, Nov 09 2006
Practical numbers first appear in Srinivasan's short paper, which contains terms up to 200. Let n be a practical number. He states that (1) if n>2, n is multiple of 4 or 6; (2) sigma(n) >= 2n-1 (A103288); and (3) 2^t n is practical. He also states that highly composite numbers (A002182), perfect numbers (A000396), and primorial numbers (A002110) are practical. - T. D. Noe, Apr 02 2010
Strengthening a theorem of Hausman and Shapiro, Pollack shows that every n > 3 for which f(n) >= sqrt{e^{gamma} n log log{n}} is a practical number, where f(n) is the largest integer such that all 0 < m < f(n) can be represented as a sum of distinct divisors of n. (By definition, n is practical if and only if f(n) >= n.) - Jonathan Vos Post, Jan 16 2012, corrected by Charles R Greathouse IV, May 10 2013
Conjecture: The sequence a(n)^(1/n) (n=3,4,...) is strictly decreasing to the limit 1. - Zhi-Wei Sun, Jan 12 2013
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REFERENCES
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H. Heller, Mathematical Buds, Vol. 1 Chap. 2 pp. 10-22, Mu Alpha Theta OK 1978.
M. R. Heyworth, More on Panarithmic Numbers. New Zealand Math. Mag. 17, 28-34 (1980) [ ISSN 0549-0510 ].
H. J. Hindin, Quasipractical numbers, IEEE Communications Magazine, March 1980, pp. 41-45.
R. 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).
A. K. Srinivasan, Practical numbers, Current Science, 17 (1948), 179-180.
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LINKS
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T. D. Noe, Table of n, a(n) for n = 1..1000
C. McLeman, PlanetMath.org, Practical number
G. Melfi, On two conjectures about practical numbers, J. Number Theory 56 (1996) 205-210 [MR96i:11106].
G. Melfi, On certain positive integer sequences, arXiv:0404555 [math.NT].
G. Melfi, A survey of practical numbers (<2008)
G. Melfi, Practical Numbers (old link)
Paul Pollack and Lola Thompson, Practical pretenders, arXiv:1201.3168v1 [math.NT], Jan 16, 2012
E. Saias, Entiers a diviseurs denses 1, J. Number Theory 62 (1) (1997) p. 163 uses this definition.
A. K. Srinivasan, Practical numbers, Current Science, 17 (1948), 179-180.
B. M. Stewart, Sums of distinct divisors, Amer. J. Math., 76 (1954), 779-785 [MR64800]
Peter Taylor, Table of n, a(n) for n = 1..1000000
Eric Weisstein's World of Mathematics, Practical Number
Wikipedia, Practical number
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FORMULA
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Saias proves that a(n) ~ n log n. - Charles R Greathouse IV, May 10 2013
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MATHEMATICA
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PracticalQ[n_] := Module[{f, p, e, prod=1, ok=True}, If[n<1 || (n>1 && OddQ[n]), False, If[n==1, True, f=FactorInteger[n]; {p, e} = Transpose[f]; Do[If[p[[i]] > 1+DivisorSigma[1, prod], ok=False; Break[]]; prod=prod*p[[i]]^e[[i]], {i, Length[p]}]; ok]]]; Select[Range[200], PracticalQ] (* T. D. Noe, Apr 02 2010 *)
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PROG
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(Haskell)
a005153 n = a005153_list !! (n-1)
a005153_list = filter (\x -> all (p $ a027750_row x) [1..x]) [1..]
where p _ 0 = True
p [] _ = False
p ds'@(d:ds) m = d <= m && (p ds (m - d) || p ds m)
-- Reinhard Zumkeller, Feb 23 2024, Oct 27 2011
(PARI) is_A005153(n)=bittest(n, 0) && return(n==1); my(P=1); n && !for(i=2, #n=factor(n)~, n[1, i]>1+(P*=sigma(n[1, i-1]^n[2, i-1])) && return) \\ - M. F. Hasler, Jan 13 2013
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CROSSREFS
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Cf. A007620 (second definition), A030057, A033630, A174533.
Cf. A027750.
Sequence in context: A103288 A125225 A092903 * A174973 A231565 A191743
Adjacent sequences: A005150 A005151 A005152 * A005154 A005155 A005156
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KEYWORD
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nonn,nice,easy,changed
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AUTHOR
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N. J. A. Sloane.
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EXTENSIONS
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More terms from Pab Ter (pabrlos(AT)yahoo.com), May 09 2004
Erroneous comment removed by T. D. Noe, Nov 14 2010
Definition changed to exclude n=0 explicitly by M. F. Hasler, Jan 19 2013
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STATUS
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approved
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