OFFSET
1,1
COMMENTS
LINKS
FORMULA
a(n) = 5*A029744(n); numbers of the forms 5*2^n and 15*2^n.
G.f.: 5*x*(x+1)^2/(1-2*x^2). - Ralf Stephan, Jul 15 2013
Sum_{n>=1} 1/a(n) = 8/15. - Amiram Eldar, Jan 02 2021
MATHEMATICA
pf[x_] := Part[Reverse[Flatten[FactorInteger[x]]], 2]; Do[s=EulerPhi[pf[n]]-pf[EulerPhi[n]]; If[Equal[s, 2], Print[n]], {n, 3, 1000000}]
Union[Flatten[Table[2^n {5, 15}, {n, 0, 20}]]] (* or *) Join[ {5}, LinearRecurrence[ {0, 2}, {10, 15}, 40]] (* Harvey P. Dale, Dec 23 2014 *)
PROG
(PARI) gpf(n)=if(n>1, my(f=factor(n)[, 1]); f[#f], 1)
is(n)=eulerphi(gpf(n))-gpf(eulerphi(n))==2 \\ Charles R Greathouse IV, Feb 19 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Labos Elemer, May 07 2002
EXTENSIONS
Simpler name by Joerg Arndt, Jul 16 2013
STATUS
approved