This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A173337 Numbers n such that phi(phi(n))= sigma(sopf(n)). 0
 40, 50, 54, 171, 195, 231, 330, 377, 387, 518, 638, 742, 745, 888, 1057, 1141, 1397, 1561, 1788, 2422, 2682, 2763, 3206, 3357, 3805, 4037, 4344, 4382, 4915, 5093, 5138, 5391, 5558, 5951, 6147, 8063, 8952, 9132, 9422, 10109, 10968, 11796, 12287, 12481 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS sopf(n) is the sum of the distinct primes dividing n (without repetition): A008472) phi(n): Euler totient function (A000010) sigma(n) = sum of divisors of n (A000203) LINKS FORMULA n such that A010554(n)= sigma(A008472(n)) EXAMPLE 40 is in the sequence because phi(40)= 16,phi(16)=8, sopf(40)=7 and sigma(7) = 8; 171 is in the sequence because phi(171)= 108, phi(108)=36, sopf(171)=22 and sigma(22)= 36. MAPLE with(numtheory) : A008472 := proc(n)         add(p, p = factorset(n): end proc: isA173337 := proc(n)         phi(phi(n)) = sigma(A008472(n)) ; end proc: for n from 1 do         if isA173337(n) then printf("%d, ", n) ; fi; end do: # R. J. Mathar, Jul 06 2012 CROSSREFS Sequence in context: A062909 A167327 A216305 * A167311 A039523 A217729 Adjacent sequences:  A173334 A173335 A173336 * A173338 A173339 A173340 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 April 23 02:15 EDT 2019. Contains 322380 sequences. (Running on oeis4.)