1,1

2 is the only number m such that sigma(m)=1.5*m.

The next term in this sequence is greater than 5*10^9.

A direct consequence of Robin's theorem is that a(6)>5E16, a(7)>1.898E29, a(8)>2.144E51, a(9)>9.877E89 and a(10)>6.023E157. [From W. Bomfim (webonfim(AT)bol.com.br), Oct 30 2008]

If the Riemann hypothesis (RH) is true then Robin's theorem (Guy Robin, 1984) implies that the n-th term of this sequence is greater than exp(exp((n+1/2)/exp(gamma))) where gamma=0.5772156649... is the Euler-Mascheroni constant (A001620). For the 6-th term (which is actually 1.7*10^44) this lower bound is 5.0*10^16. Similarly, if RH is true, the next term (7-th term) is at least 1.9*10^29 (and is probably more than 10^90 or so). [From Gerard P. Michon (g.michon(AT)att.net), Jun 10 2009]

Contribution from Gerard P. Michon (g.michon(AT)att.net), Jul 04 2009: (Start)

An upper bound for a(7) is provided by a 97-digit integer of abundancy 15/2 (5.71379...10^96) discovered by Michel Marcus on July 4, 2009. The factorization of that number is: 2^53 3^15 5^6 7^6 11^3 13 17 19^3 23 29 31 37 41 43 61 73 79 97 181 193 199 257 263 4733 11939 19531 21803 87211 262657

Similarly, an upper bound for a(8) is provided by a 286-digit integer of abundancy 17/2 (3.30181...10^285) equal to x/17, where x is the smallest known number of abundancy 9 (a 287-digit integer discovered by Fred W. Helenius in 1995). This is so because 17 happen to occur with multiplicity 1 in the factorization of x. (End)

A new upper bound for a(7) was found on August 15, 2009 by Michel Marcus who broke his own record by finding two "small" multiples of 2^35*3^20*5^5*7^6*11^2*13^2*17 that are of abundancy 15/2. The lower one (1.27494722...10^88) has only 89 digits. [From Gerard P. Michon (g.michon(AT)att.net), Aug 15 2009]

Guy Robin, Grandes valeurs de la fonction somme des diviseurs et hypothese de Riemann, J. Math. Pures Appl. 63 (1984), 187-213. [From Gerard P. Michon (g.michon(AT)att.net), Jun 10 2009]

Walter Nissen, Abundancy : Some Resources

Wikipedia, Riemann hypothesis [From W. Bomfim (webonfim(AT)bol.com.br), Oct 30 2008]

G. P. Michon, Multiplicative functions: Abundancy = sigma(n)/n [From Gerard P. Michon (g.michon(AT)att.net), Jun 10 2009]

G. P. Michon, Multiperfect and hemiperfect integers [From Gerard P. Michon (g.michon(AT)att.net), Jun 10 2009]

G. P. Michon and M. Marcus, Hemiperfect numbers of abundancy 11/2 [From Gerard P. Michon (g.michon(AT)att.net), Aug 06 2009]

G. P. Michon and M. Marcus, Hemiperfect numbers of abundancy 13/2 [From Gerard P. Michon (g.michon(AT)att.net), Aug 06 2009]

G. P. Michon and M. Marcus, Hemiperfect numbers of abundancy 15/2 [From Gerard P. Michon (g.michon(AT)att.net), Aug 06 2009]

a(2)=24 because 1+2+3+4+6+8+12+24=2.5*24 and 24 is the earliest m such that sigma(m)=2.5*m.

a[n_] := (For[m=1, DivisorSigma[1, m]!=(n+1/2)m, m++ ]; m); Do[Print[a[n]], {n, 4}]

Cf. A007539, A055153, A000396, A005820, A027687.

A141643 (abundancy = 5/2), A141645 (abundancy = 9/2), A159271 (abundancy = 11/2), A160678 (abundancy = 13/2), A159907 (half-integral abundancy, "hemiperfect numbers"). [From Gerard P. Michon (g.michon(AT)att.net), Jun 10 2009]

Sequence in context: A059332 A000794 A159907 this_sequence A055462 A088600 A066120

Adjacent sequences: A088909 A088910 A088911 this_sequence A088913 A088914 A088915

hard,more,nonn

Farideh Firoozbakht (f.firoozbakht(AT)sci.ui.ac.ir), Nov 29 2003

a(5)-a(6) from Robert Gerbicz (robert.gerbicz(AT)gmail.com), Apr 19 2009