This site is supported by donations to The OEIS Foundation.

 Annual Appeal: Please make a donation to keep the OEIS running. In 2018 we replaced the server with a faster one, added 20000 new sequences, and reached 7000 citations (often saying "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A287800 Numbers n such that phi(n) * tau(n) divides n^2, but neither tau(n) nor phi(n) divides n. 0
 900, 2400, 3840, 6480, 7200, 11520, 13056, 39168, 42240, 79200, 83232, 96000, 126720, 145200, 153600, 157440, 174240, 195840, 207360, 288000, 300000, 317520, 326592, 387840, 435600, 460800, 472320, 480000, 900000, 971520, 1056000, 1161600, 1163520, 1228800, 1440000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The GCD of the first 43 terms is 12. The GCD of the first 166 terms is 4. The GCD of a(2) through a(166) is 16. - David A. Corneth, Jun 01 2017 LINKS EXAMPLE For n = 900, tau(900) = 27, phi(900) = 240 and 900^2/(27 * 240) = 125, but 900/27 = 33.33333 and 900/240 = 3.75. MAPLE for n from 1 to 100000 do p(n):=n^2/(tau(n)*phi(n)); if p(n)=floor(p(n)) and n/tau(n)<>floor(n/tau(n)) and n/phi(n)<>floor(n/phi(n)) then print(n, p(n), phi(n), tau(n)) else fi; od: MATHEMATICA Select[Range[10^6], Function[n, And[Divisible[n^2, #1 #2], NoneTrue[{#1, #2}, Divisible[n, #] &]] & @@ {DivisorSigma[0, n], EulerPhi[n]}]] (* Michael De Vlieger, Jun 01 2017 *) PROG (PARI) is(n) = n^2 % (numdiv(n)*eulerphi(n)) == 0 && n % numdiv(n) != 0 && n % eulerphi(n) % n!=0 \\ David A. Corneth, Jun 01 2017 CROSSREFS Cf. A000005, A000010, A007694, A033950, A235353. Sequence in context: A074853 A162143 A258888 * A246887 A232556 A069096 Adjacent sequences:  A287797 A287798 A287799 * A287801 A287802 A287803 KEYWORD nonn AUTHOR Bernard Schott, Jun 01 2017 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 December 12 14:27 EST 2018. Contains 318075 sequences. (Running on oeis4.)