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A063906 Numbers n such that n = 2*sigma(n)/3 - 1. 4
15, 207, 1023, 2975, 19359, 147455, 1207359, 5017599, 2170814463, 58946212863 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Original title: numbers n such that t(n) = s(n), where s(n) = sigma(n)-n-1 and t(n) = |s(n)-n|+1.

From Robert Israel, Jan 12 2016: (Start)

All terms are odd and satisfy A009194(n) = 1 or 3.

Includes 3^(k-1)*(3^k-4) for k in A058959.

The first few terms of this form are 15, 207, 19359, 36472996363223648799.

Other terms include 3^15*43048567*1003302465131 = 619739816695811335405066239 and 3^15*43049011*808868950607 = 499643410492503517919703039. (End)

a(11) > 10^12. - Giovanni Resta, Apr 14 2016

LINKS

Table of n, a(n) for n=1..10.

Antal Bege, Kinga Fogarasi, Generalized perfect numbers, Acta Univ. Sapientiae, Math., 1 (2009), 73-82.

EXAMPLE

sigma(1207359) = 1811040; 1811040 - 1207359 - 1 = 603680; abs(603680 - 1207359) + 1 = 603680.

MAPLE

select(n -> numtheory:-sigma(n) = 3/2*(n+1), [seq(i, i=1..10^6, 2)]); # Robert Israel, Jan 12 2016

MATHEMATICA

Select[Range[10^6], 2 * DivisorSigma[1, #]/3 - 1 == # &] (* Giovanni Resta, Apr 14 2016 *)

PROG

(PARI) s(n) = sigma(n)-n-1;

t(n) = abs(s(n)-n)+1;

for(n=1, 10^8, if(t(n)==s(n), print1(n, ", ")))

(ARIBAS): for n := 1 to 4000000 do s := sigma(n) - n - 1; t := abs(s - n) + 1; if s = t then write(n, " "); end; end;

(MAGMA) [n: n in [1..6*10^6] | 2*DivisorSigma(1, n)/3-1 eq n]; // Vincenzo Librandi, Oct 10 2017

CROSSREFS

Cf. A000203, A009194, A014567, A058959.

Sequence in context: A093747 A061637 A231546 * A194481 A078265 A089138

Adjacent sequences:  A063903 A063904 A063905 * A063907 A063908 A063909

KEYWORD

nonn,more

AUTHOR

Jason Earls, Aug 30 2001

EXTENSIONS

More terms from Klaus Brockhaus, Sep 01 2001

a(9)-a(10) from Giovanni Resta, Apr 14 2016

Simpler title suggested by Giovanni Resta, Apr 14 2016, based on formula provided by Paolo P. Lava, Jan 12 2016

STATUS

approved

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Last modified September 17 13:00 EDT 2019. Contains 327131 sequences. (Running on oeis4.)