OFFSET
1,2
REFERENCES
F. Hirzebruch et al., Manifolds and Modular Forms, Vieweg, 2nd ed. 1994, p. 158.
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..10000
FORMULA
Except for n=1, 2, 4, this is A029935(n)/2.
a(n) = (1/2)*Sum_{d divides n} phi(d)*phi(n/d), with a(1)=1, a(2)=2, a(3)=2, a(4)=3, and phi(n) = A000010(n). - G. C. Greubel, Dec 13 2017
MATHEMATICA
a[1] = 1; a[2] = 2; a[4] = 3; a[n_] := DivisorSum[n, EulerPhi[#]* EulerPhi[n/#]&]/2; Array[a, 60] (* Jean-François Alcover, Oct 03 2016 *)
PROG
(PARI) for(n=1, 30, print1(if(n==1, 1, if(n==2, 2, if(n==3, 2, if(n==4, 3, sumdiv(n, d, eulerphi(d)*eulerphi(n/d))/2)))), ", ")) \\ G. C. Greubel, Dec 13 2017
CROSSREFS
KEYWORD
nonn,nice
AUTHOR
STATUS
approved