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A077445
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Numbers k such that (k^2 - 8)/2 is a square.
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7
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4, 20, 116, 676, 3940, 22964, 133844, 780100, 4546756, 26500436, 154455860, 900234724, 5246952484, 30581480180, 178241928596, 1038870091396, 6054978619780, 35291001627284, 205691031143924, 1198855185236260
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OFFSET
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1,1
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COMMENTS
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The equation "(k^2 - 8)/2 is a square" is a version of the generalized Pell Equation "x^2 - D*y^2 = C".
4*(1-z)/(1-6*z+z^2) = Sum_{n>=0} a(n+1)*z^n is the formal power series for tan(4*x)/tan(x) if one lets
z = (tan(x))^2. For the numerator and denominator of this o.g.f. see A034867 and A034839, respectively. Convergence holds for 0 <= z < 3 - 2*sqrt(2), approximately 0.1715728753. This means for |x| < Pi/8, approximately 0.3926990818.
(End)
Positive values of x (or y) satisfying x^2 - 6*x*y + y^2 + 64 = 0. - Colin Barker, Feb 13 2014
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REFERENCES
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A. H. Beiler, "The Pellian." Ch. 22 in Recreations in the Theory of Numbers: The Queen of Mathematics Entertains. Dover, New York, New York, pp. 248-268, 1966.
L. E. Dickson, History of the Theory of Numbers, Vol. II, Diophantine Analysis. AMS Chelsea Publishing, Providence, Rhode Island, 1999, pp. 341-400.
Peter G. L. Dirichlet, Lectures on Number Theory (History of Mathematics Source Series, V. 16); American Mathematical Society, Providence, Rhode Island, 1999, pp. 139-147.
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LINKS
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FORMULA
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a(n) = (((3+2*sqrt(2))^n + (3-2*sqrt(2))^n) + ((3+2*sqrt(2))^(n-1) + (3-2*sqrt(2))^(n-1))) / 2.
a(n) = 6*a(n-1) - a(n-2) = 4*A001653(n).
G.f.: 4*(x-x^2)/(1-6*x+x^2).
With a=3+2*sqrt(2), b=3-2*sqrt(2): a(n) = sqrt(2)*(a^((2*n-1)/2) + b^((2*n-1)/2)). a(n) = sqrt(2*A003499(2*n-1)+4). - Mario Catalani (mario.catalani(AT)unito.it), Mar 24 2003
a(n) = (A003499(n+1) + A003499(n))/2. - Mario Catalani (mario.catalani(AT)unito.it), Mar 31 2003
G.f.: 2*Q(0)*x*(1-x)/(1-3*x), where Q(k) = 1 + 1/( 1 - x*(8*k-9)/( x*(8*k-1) - 3/Q(k+1) )); (continued fraction). - Sergei N. Gladkovskii, Dec 10 2013
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MATHEMATICA
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CoefficientList[Series[4 (1 - x)/(1 - 6 x + x^2), {x, 0, 40}], x] (* Vincenzo Librandi, Feb 14 2014 *)
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PROG
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(PARI) a(n)=if(n<1, 0, subst(poltchebi(n)+poltchebi(n-1), x, 3))
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CROSSREFS
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KEYWORD
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nonn,easy,changed
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AUTHOR
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STATUS
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approved
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