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A173328
Numbers k such that phi(tau(k)) = tau(sopf(k)).
1
4, 6, 8, 9, 10, 12, 18, 20, 22, 25, 27, 30, 32, 34, 44, 49, 50, 58, 60, 68, 70, 82, 90, 102, 104, 105, 116, 118, 121, 125, 135, 140, 142, 150, 152, 164, 169, 174, 182, 189, 190, 195, 202, 204, 208, 214, 231, 236, 238, 242, 243, 246, 248, 252, 274, 284, 285, 286
OFFSET
1,1
COMMENTS
Sopf = A008472 is the sum of the distinct primes dividing n, tau= A000005 is the number of divisors, phi = A000010 is Euler's totient.
LINKS
P. A. MacMahon, Divisors of numbers and their continuations in the theory of partitions, Proc. London Math. Soc., 19 (1919), 75-113.
W. Sierpinski, Number Of Divisors And Their Sum, Elementary theory of numbers, Warszawa, 1964.
FORMULA
{n : A163109(n)= tau(A008472(n))}.
EXAMPLE
4 is in the sequence because tau(4) = 3, phi(3)=2, sopf(4)=2 and tau(2) = 2;
6 is in the sequence because tau(6) = 4, phi(6)=2, sopf(6)=5 and tau(5) = 2.
MAPLE
isA173328 := proc(n)
numtheory[phi](numtheory[tau](n)) = numtheory[tau](A008472(n)) ;
end proc:
for n from 1 to 300 do
if isA173328(n) then
printf("%d, ", n);
end if;
end do: # R. J. Mathar, Nov 07 2011
MATHEMATICA
Select[Range[2, 300], EulerPhi[DivisorSigma[0, #]]==DivisorSigma[0, Total[ FactorInteger[#][[All, 1]]]]&] (* Harvey P. Dale, May 30 2017 *)
CROSSREFS
Cf. A008472 (sopfr).
Sequence in context: A047820 A248807 A034878 * A116661 A109104 A073303
KEYWORD
nonn
AUTHOR
Michel Lagneau, Feb 16 2010
STATUS
approved