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A097309
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Chebyshev polynomials of the second kind, U(n,x), evaluated at x=13.
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22
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0, 1, 26, 675, 17524, 454949, 11811150, 306634951, 7960697576, 206671502025, 5365498355074, 139296285729899, 3616337930622300, 93885489910449901, 2437406399741075126, 63278680903357503375, 1642808297087554012624
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OFFSET
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-1,3
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COMMENTS
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b(n)^2 - 42*(2*a(n))^2 = +1 with b(n):=A097308(n) gives all nonnegative integer solutions of this D:=42*4=168 Pell equation.
For positive n, a(n) equals the permanent of the n X n tridiagonal matrix with 26's along the main diagonal, and i's along the superdiagonal and the subdiagonal (i is the imaginary unit). - John M. Campbell, Jul 08 2011
For n>=1, a(n) equals the number of 01-avoiding words of length n-1 on alphabet {0,1,...,25}. - Milan Janjic, Jan 25 2015
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LINKS
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FORMULA
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a(n) = S(n, 26) = U(n, 13), n >= -1, with Chebyshev polynomials of 2nd kind. See A049310 for the triangle of S(n, x) coefficients. S(-1, x) := 0 =: U(-1, x).
a(n) = ((13+2*sqrt(42))^n - (13-2*sqrt(42))^n)/(4*sqrt(42)), (Binet form).
a(n) = Sum_{k=0..floor(n/2)} (-1)^k * binomial(n-k, k) * 26^(n-2*k).
G.f.: 1/(1-26*x+x^2).
With an offset of 0, product {n >= 1} (1 + 1/a(n)) = 1/6*(6 + sqrt(42)). - Peter Bala, Dec 23 2012
Product {n >= 2} (1 - 1/a(n)) = 1/13*(6 + sqrt(42)). - Peter Bala, Dec 23 2012
a(n) = sqrt((A097308(n)^2 - 1)/168).
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MAPLE
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seq( simplify(ChebyshevU(n, 13)), n=-1..20); # G. C. Greubel, Dec 22 2019
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MATHEMATICA
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PROG
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(Sage) [lucas_number1(n, 26, 1) for n in range(0, 20)] # Zerinvary Lajos, Jun 25 2008
(Sage) [chebyshev_U(n, 13) for n in (-1..20)] # G. C. Greubel, Dec 22 2019
(PARI) vector( 22, n, polchebyshev(n-2, 2, 13) ) \\ G. C. Greubel, Dec 22 2019
(Magma) m:=13; I:=[0, 1]; [n le 2 select I[n] else 2*m*Self(n-1) -Self(n-2): n in [1..20]]; // G. C. Greubel, Dec 22 2019
(GAP) m:=13;; a:=[0, 1];; for n in [3..20] do a[n]:=2*m*a[n-1]-a[n-2]; od; a; # G. C. Greubel, Dec 22 2019
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CROSSREFS
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Chebyshev sequence U(n, m): A000027 (m=1), A001353 (m=2), A001109 (m=3), A001090 (m=4), A004189 (m=5), A004191 (m=6), A007655 (m=7), A077412 (m=8), A049660 (m=9), A075843 (m=10), A077421 (m=11), A077423 (m=12), this sequence (m=13), A097311 (m=14), A097313 (m=15), A029548 (m=16), A029547 (m=17), A144128 (m=18), A078987 (m=19), A097316 (m=33).
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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