OFFSET
1,2
FORMULA
a(n) ~ 3 * sqrt(n) * 4^n / Pi^(5/2).
MATHEMATICA
Table[Sum[Binomial[2*n, n-k]*EulerPhi[k], {k, 1, n}], {n, 1, 30}]
PROG
(PARI) a(n) = sum(k=1, n, binomial(2*n, n-k) * eulerphi(k)); \\ Michel Marcus, Aug 05 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Vaclav Kotesovec, Aug 04 2022
STATUS
approved