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A336715
Numbers m that divide the product phi(m) * tau(m), where tau is the number of divisors function (A000005) and phi is the Euler totient function (A000010).
2
1, 2, 8, 9, 12, 18, 32, 36, 72, 80, 96, 108, 128, 144, 243, 288, 324, 400, 448, 486, 512, 576, 625, 720, 768, 864, 972, 1152, 1200, 1250, 1344, 1620, 1944, 2000, 2025, 2048, 2304, 2500, 2560, 2592, 2916, 3136, 3600, 3888, 4032, 4050, 4608, 5000, 5103, 5625, 6144, 6561, 6912
OFFSET
1,2
COMMENTS
Numbers of the form q = 2^(2k+1) with k>=0 (A004171) form a subsequence because tau(q) * phi(q) / q = k + 1.
Numbers of the form q = 9 * 2^k with k>=0 (A005010) form another subsequence because tau(q) * phi(q) / q = k+1 (also).
LINKS
David A. Corneth, Table of n, a(n) for n = 1..12173 (terms <= 10^15)
EXAMPLE
For 80, phi(80) = 32, tau(80) = 10 and tau(80)*phi(80)/80 = 4, hence 80 is a term.
MAPLE
with(numtheory):
filter:= m-> irem(phi(m)*tau(m), m)=0:
select(filter, [$1..7000])[];
MATHEMATICA
Select[Range[7000], Divisible[DivisorSigma[0, #] * EulerPhi[#], #] &] (* Amiram Eldar, Aug 01 2020 *)
PROG
(PARI) isok(m) = (eulerphi(m)*numdiv(m) % m) == 0; \\ Michel Marcus, Aug 02 2020
CROSSREFS
Cf. A000010 (phi), A000005 (tau), A062355.
Subsequences: A004171, A005010.
Sequence in context: A046526 A279373 A057529 * A120737 A081381 A235524
KEYWORD
nonn
AUTHOR
Bernard Schott, Aug 01 2020
STATUS
approved