login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A091191 Primitive abundant numbers: abundant numbers (A005101) having no abundant proper divisor. 25

%I

%S 12,18,20,30,42,56,66,70,78,88,102,104,114,138,174,186,196,222,246,

%T 258,272,282,304,308,318,354,364,366,368,402,426,438,464,474,476,498,

%U 532,534,550,572,582,606,618,642,644,650,654,678,748,762,786,812,822

%N Primitive abundant numbers: abundant numbers (A005101) having no abundant proper divisor.

%C A080224(a(n)) = 1.

%C This is a supersequence of the primitive abundant number sequence A071395, since many of these numbers will be positive integer multiples of the perfect numbers (A000396). - _Timothy L. Tiffin_, Jul 15 2016

%C If the terms of A071395 are removed from this sequence, then the resulting sequence will be A275082. - _Timothy L. Tiffin_, Jul 16 2016

%H T. D. Noe, <a href="/A091191/b091191.txt">Table of n, a(n) for n = 1..10000</a>

%H P. Erdős, <a href="http://www.renyi.hu/~p_erdos/1934-04.pdf">On the density of the abundant numbers</a>, J. London Math. Soc. 9 (1934), pp. 278-282.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/AbundantNumber.html">Abundant Number</a>

%F Erdős shows that a(n) >> n log^2 n. - _Charles R Greathouse IV_, Dec 05 2012

%e 12 is a term since 1, 2, 3, 4, and 6 (the proper divisors of 12) are either deficient or perfect numbers, and thus not abundant. - _Timothy L. Tiffin_, Jul 15 2016

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

%p isA091191 := proc(n) local d; if isA005101(n) then for d in numtheory[divisors](n) minus {1,n} do if isA005101(d) then return false; end if; end do: return true; else false; end if; end proc:

%p for n from 1 to 200 do if isA091191(n) then printf("%d\n",n) ; end if;end do: # _R. J. Mathar_, Mar 28 2011

%t t = {}; n = 1; While[Length[t] < 100, n++; If[DivisorSigma[1, n] > 2*n && Intersection[t, Divisors[n]] == {}, AppendTo[t, n]]]; t (* _T. D. Noe_, Mar 28 2011 *)

%t Select[Range@ 840, DivisorSigma[1, #] > 2 # && Times @@ Boole@ Map[DivisorSigma[1, #] <= 2 # &, Most@ Divisors@ #] == 1 &] (* _Michael De Vlieger_, Jul 16 2016 *)

%o (PARI) is(n)=sumdiv(n,d,sigma(d,-1)>2)==1 \\ _Charles R Greathouse IV_, Dec 05 2012

%o (Haskell)

%o a091191 n = a091191_list !! (n-1)

%o a091191_list = filter f [1..] where

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

%o where pdivs = a027751_row x

%o -- _Reinhard Zumkeller_, Jan 31 2014

%Y Cf. A006038 (odd terms), A005101 (abundant numbers), A091192.

%Y Cf. A027751, A071395 (subsequence), supersequence of A275082.

%Y Cf. A294930 (characteristic function), A294890.

%K nonn

%O 1,1

%A _Reinhard Zumkeller_, Dec 27 2003

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified April 22 10:58 EDT 2019. Contains 322330 sequences. (Running on oeis4.)