The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 (list; graph; refs; listen; history; text; internal format)
 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 Harvey P. Dale, Table of n, a(n) for n = 1..1000 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 Adjacent sequences:  A173325 A173326 A173327 * A173329 A173330 A173331 KEYWORD nonn AUTHOR Michel Lagneau, Feb 16 2010 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified August 10 08:35 EDT 2020. Contains 336368 sequences. (Running on oeis4.)