A324707 - OEIS

A324707

Tri-unitary perfect numbers: numbers k such that tsigma(k) = 2k, where tsigma(k) is the sum of the tri-unitary divisors of k (A324706).

6, 60, 90, 36720, 47520, 8173440, 22276800, 126463680, 597542400, 4201148160, 287704872000, 1632485836800

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OFFSET

1,1

COMMENTS

Also in the sequence is 21623407345626345971712000.

a(13) > 5*10^12. - Giovanni Resta, Mar 14 2019

LINKS

Table of n, a(n) for n=1..12.

EXAMPLE

36720 is in the sequence since its sum of tri-unitary divisors is A324706(36720) = 73440 = 2 * 36720.

MATHEMATICA

f[p_, e_] := If[e == 3, (p^4-1)/(p-1), If[e==6, (p^8-1)/(p^2-1), p^e+1]]; tsigma[1]=1; tsigma[n_]:= Times @@ f @@@ FactorInteger[n]; Select[Range[50000], tsigma[#]==2# &]

CROSSREFS

Cf. A000396, A002827, A007357, A322486, A324706, A324708, A324709.

Sequence in context: A322486 A323757 A331108 * A007357 A327158 A002827

Adjacent sequences: A324704 A324705 A324706 * A324708 A324709 A324710

KEYWORD

nonn,more

AUTHOR

Amiram Eldar, Mar 11 2019

EXTENSIONS

a(11)-a(12) from Giovanni Resta, Mar 14 2019

STATUS

approved