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FORMULA
| a(n)= sum(S(k, 20), k=0..n) with S(k, 20)=U(k, 10)=A075843(k+1) Chebyshev's polynomials of the second kind.
G.f.: 1/((1-x)*(1-20*x+x^2)) = 1/(1-21*x+21*x^2-x^3).
a(n)=20*a(n-1)-a(n-2)+1, n>=1, a(-1):=0, a(0)=1.
a(n)= (S(n+1, 20) - S(n, 20) -1)/18.
a(n)=21*a(n-1)-21*a(n-2)+a(n-3), n>=2, a(-1):=0, a(0)=1, a(1)=21.
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