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A006038 Odd primitive abundant numbers.
(Formerly M5486)
21
945, 1575, 2205, 3465, 4095, 5355, 5775, 5985, 6435, 6825, 7245, 7425, 8085, 8415, 8925, 9135, 9555, 9765, 11655, 12705, 12915, 13545, 14805, 15015, 16695, 18585, 19215, 19635, 21105, 21945, 22365, 22995, 23205, 24885, 25935, 26145, 26565, 28035, 28215 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Dickson proves that there are only a finite number of odd primitive abundant numbers having n distinct prime factors. Sequence A188342 lists the smallest such numbers. - T. D. Noe, Mar 28 2011

Sequence A188439 sorts the numbers in this sequence by the number of distinct prime factors. Eight numbers have exactly three prime factors; 576 have exactly four prime factors. - T. D. Noe, Apr 04 2011

Any multiple of an abundant number (A005101) is again an abundant number. Primitive abundant numbers (A091191) are those not of this form, i.e., without an abundant proper divisor. We don't know any odd perfect number (A000396), so the (odd) terms here have only deficient proper divisors (A071395), and their prime factors p are less than sigma(n/p)/deficiency(n/p). See A005231 (odd abundant numbers) for an explanation why all terms have at least 3 distinct prime factors, and 5 prime factors when counted with multiplicity (A001222), whence a(1) = 3^3*5*7. All known terms are semiperfect (A005835, and thus in A006036): no odd weird number (A006037) is known, but if it exists, the smallest one is in this sequence. - M. F. Hasler, Jul 28 2016

REFERENCES

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

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

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.

Eric W. Weisstein, MathWorld: Primitive Abundant Number

MAPLE

isA005101 := proc(n) is(numtheory[sigma](n) > 2*n ); end proc:

isA005100 := proc(n) is(numtheory[sigma](n) < 2*n ); end proc:

isA006038 := proc(n) local d; if type(n, 'odd') and isA005101(n) then for d in numtheory[divisors](n) minus {1, n} do if not isA005100(d) then return false; end if; end do: return true; else false; end if; end proc:

n := 1 ; for a from 1 by 2 do if isA006038(a) then printf("%d %d\n", n, a) ; n := n+1 ; end if; end do: # R. J. Mathar, Mar 28 2011

MATHEMATICA

t = {}; n = 1; While[Length[t] < 50, n = n + 2; If[DivisorSigma[1, n] > 2 n && Intersection[t, Divisors[n]] == {}, AppendTo[t, n]]]; t (* T. D. Noe, Mar 28 2011 *)

PROG

(PARI) is(n)=n%2 && sumdiv(n, d, sigma(d, -1)>2)==1 \\ Charles R Greathouse IV, Jun 10 2013

(PARI) is_A006038(n)=bittest(n, 0) && sigma(n)>2*n && !for(i=1, #f=factor(n)[, 1], sigma(n\f[i], -1)>2&&return) \\ More than 5 times faster. - M. F. Hasler, Jul 28 2016

(Haskell)

a006038 n = a006038_list !! (n-1)

a006038_list = filter f [1, 3 ..] where

   f x = sum pdivs > x && all (<= 0) (map (\d -> a000203 d - 2 * d) pdivs)

         where pdivs = a027751_row x

-- Reinhard Zumkeller, Jan 31 2014

CROSSREFS

Cf. A005101, A005231. Subsequence of A091191.

Cf. A000203, A027751.

Sequence in context: A174865 A174535 A243104 * A188439 A275472 A275066

Adjacent sequences:  A006035 A006036 A006037 * A006039 A006040 A006041

KEYWORD

nonn

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified December 8 09:03 EST 2016. Contains 278906 sequences.