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FORMULA
| a(n)= sum(S(k, 19), k=0..n) with S(k, 19)=U(k, 19/2)=A078368(k) Chebyshev's polynomials of the second kind.
G.f.: 1/((1-x)*(1-19*x+x^2)) = 1/(1-20*x+20*x^2-x^3).
a(n) = 20*a(n-1)-20*a(n-2)+a(n-3), n>=2, a(-1):=0, a(0)=1, a(1)=20.
a(n) = 19*a(n-1)-a(n-2)+1, n>=1, a(-1):=0, a(0)=1.
a(n) = (S(n+1, 19) - S(n, 19) -1)/17.
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