OFFSET
1,1
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..100
MATHEMATICA
T[n_, k_] := (1/(n*k))*Sum[Sum[EulerPhi[c]*EulerPhi[d]*5^(n*k/LCM[c, d]), {d, Divisors[k]}], {c, Divisors[n]}]; a[n_] := T[n, 3]; Array[a, 14] (* Jean-François Alcover, Nov 01 2017, after Andrew Howroyd *)
PROG
(PARI) a(n) = my(k=3); (1/(n*k)) * sumdiv(n, c, sumdiv(k, d, eulerphi(c) * eulerphi(d) * 5^(n*k/lcm(c, d)))); \\ Michel Marcus, Nov 01 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Jan 10 2011
EXTENSIONS
Terms a(8) and beyond from Andrew Howroyd, Sep 27 2017
STATUS
approved