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A004394 Superabundant [or super-abundant] numbers: n such that sigma(n)/n > sigma(m)/m for all m < n, sigma(n) being A000203(n), the sum of the divisors of n. 89
1, 2, 4, 6, 12, 24, 36, 48, 60, 120, 180, 240, 360, 720, 840, 1260, 1680, 2520, 5040, 10080, 15120, 25200, 27720, 55440, 110880, 166320, 277200, 332640, 554400, 665280, 720720, 1441440, 2162160, 3603600, 4324320, 7207200, 8648640, 10810800, 21621600 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Matthew Conroy points out that these are different from the highly composite numbers - see A002182. Jul 10 1996

With respect to the comment above, neither sequence is subsequence of the other. - Ivan N. Ianakiev, Feb 11 2020

Also n such that sigma_{-1}(n) > sigma_{-1}(m) for all m < n, where sigma_{-1}(n) is the sum of the reciprocals of the divisors of n. - Matthew Vandermast, Jun 09 2004

Ramanujan (1997, Section 59; written in 1915) called these numbers "generalized highly composite." Alaoglu and Erdős (1944) changed the terminology to "superabundant." - Jonathan Sondow, Jul 11 2011

Alaoglu and Erdős show that: (1) n is superabundant => n=2^{e_2} * 3^{e_3} * ...* p^{e_p}, with e_2 >= e_3 >= ... >= e_p (and e_p is 1 unless n=4 or n=36); (2) if q < r are primes, then | e_r - floor(e_q*log(q)/log(r)) | <= 1; (3) q^{e_q} < 2^{e_2+2} for primes q, 2 < q <= p. - Keith Briggs, Apr 26 2005

It follows from Alaoglu and Erdős finding 1 (above) that, for n > 7, a(n) is a Zumkeller Number (A083207); for details, see Proposition 9 and Corollary 5 at Rao/Peng link (below). - Ivan N. Ianakiev, Feb 11 2020

See A166735 for superabundant numbers that are not highly composite, and A189228 for superabundant numbers that are not colossally abundant.

Pillai called these numbers "highly abundant numbers of the 1st order". - Amiram Eldar, Jun 30 2019

REFERENCES

R. Honsberger, Mathematical Gems, M.A.A., 1973, p. 112.

J. Sandor, "Abundant numbers", In: M. Hazewinkel, Encyclopedia of Mathematics, Supplement III, Kluwer Acad.Publ., 2002 (see pp. 19-21).

D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, 128.

LINKS

T. D. Noe and D. Kilminster, Table of n, a(n) for n = 1..2000 (First 500 terms from T. D. Noe. Extends to n=8436 in the comments.)

A. Akbary and Z. Friggstad, Superabundant numbers and the Riemann hypothesis, Amer. Math. Monthly, 116 (2009), 273-275.

L. Alaoglu and P. Erdős, On highly composite and similar numbers, Trans. Amer. Math. Soc., 56 (1944), 448-469. Errata

Yu. Bilu, P. Habegger, L. Kühne, Effective bounds for singular units, arXiv:1805.07167 [math.NT], 2018.

Benjamin Braun, Brian Davis, Antichain Simplices, arXiv:1901.01417 [math.CO], 2019.

Keith Briggs, Abundant numbers and the Riemann Hypothesis, Experimental Math., Vol. 16 (2006), p. 251-256.

Tibor Burdette and Ian Stewart, Counterexamples to a Conjecture by Alaoglu and Erdos, arXiv:2009.03306 [math.NT], 2020.

Geoffrey Caveney, Jean-Louis Nicolas and Jonathan Sondow, Robin's theorem, primes, and a new elementary reformulation of the Riemann Hypothesis, INTEGERS 11 (2011), #A33.

G. Caveney, J.-L. Nicolas and J. Sondow, On SA, CA, and GA numbers, arXiv preprint arXiv:1112.6010 [math.NT], 2011. - From N. J. A. Sloane, Apr 14 2012

G. Caveney, J.-L. Nicolas and J. Sondow, On SA, CA, and GA numbers, Ramanujan J., 29 (2012), 359-384.

P. Erdős & J.-L. Nicolas, Répartition des nombres superabondants (Text in French), Bulletin de la S. M. F., tome 103 (1975), p. 65-90.

F. Jokar, On k-layered numbers and some labeling related to k-layered numbers, arXiv:2003.11309 [math.NT], 2020.

Stepan Kochemazov, Oleg Zaikin, Eduard Vatutin, Alexey Belyshev, Enumerating Diagonal Latin Squares of Order Up to 9, J. Int. Seq., Vol. 23 (2020), Article 20.1.2.

J. C. Lagarias, An elementary problem equivalent to the Riemann hypothesis, Am. Math. Monthly 109 (#6, 2002), 534-543.

A. Morkotun, On the increase of Gronwall function value at the multiplication of its argument by a prime, arXiv preprint arXiv:1307.0083 [math.NT], 2013.

S. Nazardonyavi and S. Yakubovich, Superabundant numbers, their subsequences and the Riemann hypothesis, arXiv preprint arXiv:1211.2147 [math.NT], 2012.

S. Nazardonyavi and S. Yakubovich, Delicacy of the Riemann hypothesis and certain subsequences of superabundant numbers, arXiv preprint arXiv:1306.3434 [math.NT], 2013.

S. Nazardonyavi, S. Yakubovich, Extremely Abundant Numbers and the Riemann Hypothesis, Journal of Integer Sequences, 17 (2014), Article 14.2.8.

Walter Nissen, Abundancy : Some Resources

T. D. Noe, First 500 superabundant numbers

T. D. Noe, First 1000000 superabundant numbers (21 MB, zipped)

S. Sivasankaranarayana Pillai, Highly abundant numbers, Bulletin of the Calcutta Mathematical Society, Vol. 35,  No. 1 (1943), pp. 141-156.

S. Sivasankaranarayana Pillai, On numbers analogous to highly composite numbers of Ramanujan, Rajah Sir Annamalai Chettiar Commemoration Volume, ed. Dr. B. V. Narayanaswamy Naidu, Annamalai University, 1941, pp. 697-704.

S. Ramanujan, Highly composite numbers, Annotated and with a foreword by J.-L. Nicolas and G. Robin, Ramanujan J., 1 (1997), 119-153.

K. P. S. Bhaskara Rao and Yuejian Peng, On Zumkeller Numbers, Journal of Number Theory, Volume 133, Issue 4, April 2013, pp. 1135-1155.

T. Schwabhäuser, Preventing Exceptions to Robin's Inequality, arXiv preprint arXiv:1308.3678 [math.NT], 2013.

Eric Weisstein's World of Mathematics, Superabundant Number

Wikipedia, Superabundant number

FORMULA

a(n+1) <= 2*a(n). - A.H.M. Smeets, Jul 10 2021

MATHEMATICA

a=0; Do[b=DivisorSigma[1, n]/n; If[b>a, a=b; Print[n]], {n, 1, 10^7}]

(* Second program: convert all 8436 terms in b-file into a list of terms: *)

f[w_] := Times @@ Flatten@ {Complement[#1, Union[#2, #3]], Product[Prime@ i, {i, PrimePi@ #}] & /@ #2, Factorial /@ #3} & @@ ToExpression@ {StringSplit[w, _?(! DigitQ@ # &)], StringCases[w, (x : DigitCharacter ..) ~~ "#" :> x], StringCases[w, (x : DigitCharacter ..) ~~ "!" :> x]}; Map[Which[StringTake[#, 1] == {"#"}, f@ Last@ StringSplit@ Last@ #, StringTake[#, 1] == {}, Nothing, True, ToExpression@ StringSplit[#][[1, -1]]] &, Drop[Import["b004394.txt", "Data"], 3] ] (* Michael De Vlieger, May 08 2018 *)

PROG

(PARI) print1(r=1); forstep(n=2, 1e6, 2, t=sigma(n, -1); if(t>r, r=t; print1(", "n))) \\ Charles R Greathouse IV, Jul 19 2011

CROSSREFS

Almost the same as A077006.

The colossally abundant numbers A004490 are a subsequence, as are A023199.

Subsequence of A025487; apart from a(3) = 4 and a(7) = 36, a subsequence of A102750.

Cf. A000203, A002093, A002182.

Cf. A112974 (number of superabundant numbers between colossally abundant numbers).

Cf. A091901 (Robin's inequality), A189686 (superabundant and the reverse of Robin's inequality), A192884 (non-superabundant and the reverse of Robin's inequality).

Sequence in context: A340840 A077006 A166981 * A189686 A355304 A137425

Adjacent sequences:  A004391 A004392 A004393 * A004395 A004396 A004397

KEYWORD

nonn,nice

AUTHOR

Matthew Conroy

EXTENSIONS

Name edited by Peter Munn, Mar 13 2019

STATUS

approved

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Last modified September 28 02:47 EDT 2022. Contains 357063 sequences. (Running on oeis4.)