146301domain: NAppears in sequencesa(n) = (-1)^(n+1)*Sum_{k=0..n+1}(-1)^k*binomial(2*k,k).at n=9A054108Expansion of x/((1-x)*sqrt(1+4*x^2)).at n=21A104551Expansion of x/((1-x)*sqrt(1+4*x^2)).at n=22A104551a(1) = a(2) = 1; a(n) = Sum_{1 < k < n, k not dividing n} a(k).at n=20A307856