OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..[n/2]} Sum_{d|(n-k)} phi((n-k)/d)*C(d,k) for n>0, with a(0)=1.
MATHEMATICA
a[0] = 1; a[n_] := Sum[DivisorSum[n-k, EulerPhi[(n-k)/#] * Binomial[#, k] &], {k, 0, Floor[n/2]}]; Array[a, 50, 0] (* Amiram Eldar, Aug 15 2023 *)
PROG
(PARI) a(n)=if(n==0, 1, sum(k=0, n\2, sumdiv(n-k, d, eulerphi((n-k)/d)*binomial(d, k))))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Aug 23 2006
STATUS
approved