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A098262
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First differences of Chebyshev polynomials S(n,627)=A098260(n) with Diophantine property.
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4
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1, 626, 392501, 246097501, 154302740626, 96747572275001, 60660573513685001, 38034082845508220626, 23847309283560140647501, 14952224886709362677762501, 9375021156657486838816440626
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OFFSET
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0,2
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COMMENTS
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(25*b(n))^2 - 629*a(n)^2 = -4 with b(n)=A098261(n) give all positive solutions of this Pell equation.
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LINKS
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FORMULA
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a(n) = ((-1)^n)*S(2*n, 25*i) with the imaginary unit i and the S(n, x) = U(n, x/2) Chebyshev polynomials.
G.f.: (1-x)/(1-627*x+x^2).
a(n) = S(n, 627) - S(n-1, 627) = T(2*n+1, sqrt(629)/2)/(sqrt(629)/2), with S(n, x) = U(n, x/2) Chebyshev's polynomials of the second kind, A049310. S(-1, x) = 0 = U(-1, x) and T(n, x) Chebyshev's polynomials of the first kind, A053120.
a(n) = 627*a(n-1) - a(n-2), n > 1; a(0)=1, a(1)=626. - Philippe Deléham, Nov 18 2008
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EXAMPLE
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All positive solutions of Pell equation x^2 - 629*y^2 = -4 are (25=25*1,1), (15700=25*628,626), (9843875=25*393755,392501), (6172093925=25*246883757,246097501), ...
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MATHEMATICA
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LinearRecurrence[{627, -1}, {1, 626}, 20] (* G. C. Greubel, Aug 01 2019 *)
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PROG
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(PARI) my(x='x+O('x^20)); Vec((1-x)/(1-627*x+x^2)) \\ G. C. Greubel, Aug 01 2019
(Magma) I:=[1, 626]; [n le 2 select I[n] else 627*Self(n-1) - Self(n-2): n in [1..20]]; // G. C. Greubel, Aug 01 2019
(Sage) ((1-x)/(1-627*x+x^2)).series(x, 20).coefficients(x, sparse=False) # G. C. Greubel, Aug 01 2019
(GAP) a:=[1, 626];; for n in [3..20] do a[n]:=627*a[n-1]-a[n-2]; od; a; # G. C. Greubel, Aug 01 2019
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CROSSREFS
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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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