%I
%S 5,209,3905,64769,1045505,16764929,268386305,4294770689,68718690305,
%T 1099508482049,17592173461505,281474926379009,4503599426043905,
%U 72057593232621569,1152921501385621505,18446744060824649729,295147905127813218305,4722366482663486783489
%N a(n) = 4th Chebyshev polynomial (second kind) evaluated at 2^n.
%H Colin Barker, <a href="/A020541/b020541.txt">Table of n, a(n) for n = 0..829</a>
%H <a href="/index/Ch#Cheby">Index entries for sequences related to Chebyshev polynomials.</a>
%H <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (21,84,64).
%F a(n) = 16^(n+1)  3*4^(n+1) + 1.
%F a(n) = 21*a(n1)  84*a(n2) + 64*a(n3) for n>2.  _Colin Barker_, May 03 2015
%F G.f.: (64*x^2104*x5) / ((x1)*(4*x1)*(16*x1)).  _Colin Barker_, May 03 2015
%p with(orthopoly):seq(U(4,2^i),i=0..24);
%t Table[ChebyshevU[4,2^n],{n,1,40}] (* _Vladimir Joseph Stephan Orlovsky_, Nov 03 2009 *)
%o (PARI) Vec((64*x^2104*x5)/((x1)*(4*x1)*(16*x1)) + O(x^100)) \\ _Colin Barker_, May 03 2015
%o (PARI) a(n) = polchebyshev(4, 2, 2^n) \\ _Michel Marcus_, May 03 2015
%K nonn,easy
%O 0,1
%A _Simon Plouffe_
