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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). 7
6, 60, 90, 36720, 47520, 8173440, 22276800, 126463680, 597542400, 4201148160, 287704872000, 1632485836800 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Also in the sequence is 21623407345626345971712000.
a(13) > 5*10^12. - Giovanni Resta, Mar 14 2019
LINKS
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
Sequence in context: A322486 A323757 A331108 * A007357 A327158 A002827
KEYWORD
nonn,more
AUTHOR
Amiram Eldar, Mar 11 2019
EXTENSIONS
a(11)-a(12) from Giovanni Resta, Mar 14 2019
STATUS
approved

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Last modified May 30 13:17 EDT 2023. Contains 363050 sequences. (Running on oeis4.)