login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A063530 Numbers k such that phi(k)+1 is a square. 3
15, 16, 20, 24, 30, 35, 39, 45, 52, 56, 65, 70, 72, 78, 84, 90, 104, 105, 112, 123, 130, 140, 143, 144, 155, 156, 164, 165, 168, 175, 176, 180, 183, 200, 203, 210, 215, 220, 225, 231, 244, 245, 246, 248, 261, 264, 286, 300, 308, 310, 323, 330, 339, 344, 350 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
FORMULA
A000010(k) = -1 + m^2 for some m.
EXAMPLE
If k = p*(p+2), a product of twin primes (from A037074), then k is in the sequence. The corresponding square is p^2. Other solutions are k = {56,72,78,84}, since phi(k) + 1 = 25 for all. Also phi(123) + 1 = 9^2, the square of a composite.
MATHEMATICA
Select[Range[400], IntegerQ[Sqrt[1+EulerPhi[#]]]&] (* Harvey P. Dale, Jul 31 2020 *)
PROG
(PARI) { n=0; for (a=1, 10^9, if (issquare(eulerphi(a) + 1), write("b063530.txt", n++, " ", a); if (n==1000, break)) ) } \\ Harry J. Smith, Aug 25 2009
CROSSREFS
Sequence in context: A241750 A039689 A332515 * A160661 A342941 A046417
KEYWORD
nonn
AUTHOR
Labos Elemer, Aug 02 2001
STATUS
approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 29 06:57 EDT 2024. Contains 371265 sequences. (Running on oeis4.)