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A006037 Weird numbers: abundant (A005101) but not pseudoperfect (A005835).
(Formerly M5339)
20
70, 836, 4030, 5830, 7192, 7912, 9272, 10430, 10570, 10792, 10990, 11410, 11690, 12110, 12530, 12670, 13370, 13510, 13790, 13930, 14770, 15610, 15890, 16030, 16310, 16730, 16870, 17272, 17570, 17990, 18410, 18830, 18970, 19390, 19670 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

OProject@Home in subproject Weird Engine calculates and stores the weird numbers.

There are no odd weird numbers < 10^17. - Robert A. Hearn (rah(AT)ai.mit.edu), May 25 2005

From Alois P. Heinz, Oct 30 2009: (Start)

The first weird number that has more than one decomposition of its divisors set into two subsets with equal sum (and thus is not a member of A083209) is 10430:

  1+5+7+10+14+35+298+10430 = 2+70+149+745+1043+1490+2086+5215

  2+70+298+10430 = 1+5+7+10+14+35+149+745+1043+1490+2086+5215 (end)

There are no odd weird numbers < 1.8*10^19. - Wenjie Fang, Sep 04 2013

S. Benkowski and P. Erdős (1974) proved that the asymptotic density W of weird numbers is positive. It can be shown that W < 0.0101 (see A005835). - Jaycob Coleman, Oct 26 2013

No odd weird number exists below 10^21. The search is done on the volunteer computing project yoyo@home. - Wenjie Fang, Feb 23 2014

REFERENCES

J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 70, p. 24, Ellipses, Paris 2008.

R. K. Guy, Unsolved Problems in Number Theory, B2.

H. J. Hindin, Quasipractical numbers, IEEE Communications Magazine, March 1980, pp. 41-45.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

L. Swierczewski and Donovan Johnson, Table of n, a(n) for n = 1..10000 (first 4901 terms from L. Swierczewski)

S. Benkoski, Are All Weird Numbers Even?, Problem E2308, Amer. Math. Monthly, 79 (7) (1972), 774

S. J. Benkoski and P. Erdős, On weird and pseudoperfect numbers, Math. Comp., 28 (1974), pp. 617-623. Alternate link; 1975 corrigendum

David Eppstein, Eqyptian Fractions

OProject, Weird numbers list

J. Sandor and B. Crstici, Handbook of number theory II, chapter 1.8.

Eric Weisstein's World of Mathematics, Weird Number

Wikipedia, Weird number

MATHEMATICA

(* first do *) Needs["DiscreteMath`Combinatorica`"] (* then *) fQ[n_] := Block[{d, l, t, i}, If[ DivisorSigma[1, n] > 2n && Mod[n, 6] != 0, d = Take[Divisors[n], {1, -2}]; l = 2^Length[d]; t = Table[ NthSubset[j, d], {j, l - 1}]; i = 1; While[i < l && Plus @@ t[[i]] != n, i++ ]]; If[i == l, True, False]]; Select[ Range[ 20000], fQ[ # ] &] (* from Robert G. Wilson v, May 20 2005 *)

PROG

(PARI) is_A006037(n, d=0)={ local(t); /* if d is not given, return nonzero iff n is weird ; if d is given, return nonzero iff n is not the sum of a subset of d */ if( !d, sigma(n)<=2*n & return /*must be abundant*/; d=vecextract(divisors(n), "^-1")); setsearch( Set(d), n ) & return /* equal to one element of d */; while( d[ #d]>n, d=vecextract(d, "^-1")); n >= (t = sum(i=1, #d, d[i])) & return( n-t /* nonzero if n>t */ ); n > d[ #d] & ! is_A006037( n - d[ #d], d=vecextract( d, "^-1" )) & return; is_A006037( n, d )}

(PARI) t=0; A006037=vector(100, i, until( is_A006037(t+=2), ); t) \\ - M. F. Hasler, Mar 30 2008

(Haskell)

a006037 n = a006037_list !! (n-1)

a006037_list = filter ((== 0) . a210455) a005101_list

-- Reinhard Zumkeller, Jan 21 2013

CROSSREFS

Cf. A002975, A005101, A005835, A005100, A138850, A087167.

Cf. A210455.

Sequence in context: A212236 A177298 A230897 * A002975 A251933 A061170

Adjacent sequences:  A006034 A006035 A006036 * A006038 A006039 A006040

KEYWORD

nonn,nice

AUTHOR

N. J. A. Sloane.

EXTENSIONS

More terms from Jud McCranie, Oct 21 2001

STATUS

approved

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Last modified December 21 09:07 EST 2014. Contains 252300 sequences.