login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo

Thanks to everyone who made a donation during our annual appeal!
To see the list of donors, or make a donation, see the OEIS Foundation home page.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A067786 Numbers n such that phi(n) = 2*pi(n) where pi(n) = A000720(n) and phi(n) = A000010(n). 0
11, 13, 27, 39, 63, 122, 124, 136, 152, 176, 224, 322, 364, 410, 460, 1086, 1164, 3432, 3612, 12012 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Next term > 10^6. - Michel Marcus, Nov 22 2013

There are no more terms < 10^10. Conjectured to be complete. - Donovan Johnson, Nov 24 2013

LINKS

Table of n, a(n) for n=1..20.

EXAMPLE

pi(11) = 5 and phi(11) = 10, so 11 is in the sequence.

pi(12) = 5 but phi(12) = 4, so 12 is not in the sequence.

pi(13) = 6 and phi(13) = 12, so 13 is in the sequence.

MATHEMATICA

Select[Range[500000], 2PrimePi[#] == EulerPhi[#] &] (* Harvey P. Dale, Nov 22 2013 *)

PROG

(PARI) isok(n) = eulerphi(n) == 2*primepi(n); \\ Michel Marcus, Nov 22 2013

CROSSREFS

Sequence in context: A295340 A153055 A146915 * A132245 A140567 A232880

Adjacent sequences:  A067783 A067784 A067785 * A067787 A067788 A067789

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Feb 06 2002

EXTENSIONS

Corrected and extended by Michel Marcus, Nov 22 2013

Confirmed by Harvey P. Dale, Nov 22 2013

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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified January 18 21:06 EST 2021. Contains 340262 sequences. (Running on oeis4.)