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A005231 Odd abundant numbers (odd numbers n whose sum of divisors exceeds 2n). 30
945, 1575, 2205, 2835, 3465, 4095, 4725, 5355, 5775, 5985, 6435, 6615, 6825, 7245, 7425, 7875, 8085, 8415, 8505, 8925, 9135, 9555, 9765, 10395, 11025, 11655, 12285, 12705, 12915, 13545, 14175, 14805, 15015, 15435, 16065, 16695, 17325, 17955 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

While the first even abundant number is 12 = 2^2*3, the first odd abundant is 945 = 3^3*5*7, the 232nd abundant number.

Schiffman notes that 945*630k is in this sequence for all k < 52. Most of the first initial terms are of the form. Among the 1996 terms below 10^6, 1164 terms are of that form, and only 26 terms are not divisible by 5, cf. A064001. - M. F. Hasler, Jul 16 2016

From M. F. Hasler, Jul 28 2016: (Start)

Any multiple of an abundant number is again abundant, see A006038 for primitive terms, i.e., those which are not a multiple of an earlier term.

An odd abundant number must have at least 3 distinct prime factors, and 5 prime factors when counted with multiplicity (A001222), whence a(1) = 3^3*5*7. To see this, write the relative abundancy A(N) = sigma(N)/2N = sigma[-1](N)/2 as A(Product p_i^e_i) = 1/2 Product (p_i-1/p_i^e_i)/(p_i-1) < 1/2 Product p_i/(p_i-1).

See A115414 for terms not divisible by 3, A064001 for terms not divisible by 5, A112640 for terms coprime to 5*7, and A047802 for other generalizations.

As of today, we don't know a difference between this set S of odd abundant numbers and the set S' of odd semiperfect numbers: Elements of S' \ S would be perfect (A000396), and elements of S \ S' would be weird (A006037), but no odd weird or perfect number is known. (End)

REFERENCES

W. Dunham, Euler: The Master of Us All, The Mathematical Association of America Inc., Washington, D.C., 1999, p. 13.

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

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

Jill Britton, Perfect Number Analyzer.

L. E. Dickson, Finiteness of the odd perfect and primitive abundant numbers with n distinct prime factors, American Journal of Mathematics 35 (1913), pp. 413-422.

Walter Nissen, Abundancy : Some Resources

Jay L. Schiffman, Odd Abundant Numbers, Mathematical Spectrum, Volume 37, Number 2 (January 2005), pp 73-75.

Jay L. Schiffman and Christopher S. Simons, More Odd Abundant Sequences, Volume 38, Number 1 (September 2005), pp. 7-8.

MAPLE

A005231 := proc(n) option remember ; local a ; if n = 1 then 945 ; else for a from procname(n-1)+2 by 2 do if numtheory[sigma](a) > 2*a then return a; end if; end do: end if; end proc: # R. J. Mathar, Mar 20 2011

MATHEMATICA

fQ[n_] := DivisorSigma[1, n] > 2n; Select[1 + 2Range@ 9000, fQ] (* Robert G. Wilson v, Mar 20 2011 *)

PROG

(PARI) je=[]; forstep(n=1, 15000, 2, if(sigma(n)>2*n, je=concat(je, n))); je

(PARI) is_A005231(n)={bittest(n, 0)&&sigma(n)>2*n} \\ M. F. Hasler, Jul 28 2016

CROSSREFS

Cf. A006038, A115414, A064001, A112640, A122036, A136446, A005101, A173490, A039725.

Sequence in context: A125013 A252192 A252185 * A174865 A174535 A243104

Adjacent sequences:  A005228 A005229 A005230 * A005232 A005233 A005234

KEYWORD

nonn

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from James A. Sellers

STATUS

approved

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Last modified August 27 14:48 EDT 2016. Contains 275906 sequences.