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 A077446 Numbers n such that 2*n^2 + 14 is a square. 5
 1, 5, 11, 31, 65, 181, 379, 1055, 2209, 6149, 12875, 35839, 75041, 208885, 437371, 1217471, 2549185, 7095941, 14857739, 41358175, 86597249, 241053109, 504725755, 1404960479, 2941757281, 8188709765, 17145817931, 47727298111 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The equation "2*n^2 + 14 is a square" is a version of the generalized Pell Equation x^2 - D*y^2 = C where x^2 - 2*y^2 = 14. Numbers n such that (ceiling(sqrt(n*n/2)))^2 = (7+n^2)/2. - Ctibor O. Zizka, Nov 09 2009 From Wolfdieter Lang, Feb 26 2015: (Start) This sequence gives all positive solutions x = a(n+1), n >= 0, of the Pell equation x^2 - 2*y^2 = -7. For the corresponding y-solutions see y(n) = 2*A006452(n+2) = A077447(n+1)/2. This implies that X^2 - 2*Y^2 = 14 has the general solutions (X(n),Y(n)) = (2*y(n), x(n)). See the first comment above. For the positive first class solutions see (A054490(n), 2*A038723(n)) and for the second class solutions (A255236(n), 2*A038725(n+1)). (End) REFERENCES 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. LINKS Michael De Vlieger, Table of n, a(n) for n = 1..2612 Jeremiah Bartz, Bruce Dearden, Joel Iiams, Classes of Gap Balancing Numbers, arXiv:1810.07895 [math.NT], 2018. J. J. O'Connor and E. F. Robertson, Pell's Equation Eric Weisstein's World of Mathematics, ; Pell Equation Index entries for linear recurrences with constant coefficients, signature (0,6,0,-1). FORMULA Lim. n-> Inf. a(n)/a(n-2) = 5.8284271247461... = 3 + 2*sqrt(2) = A156035 = RG (Great Ratio). Lim. k-> Inf. a(2*k+1)/a(2*k) = 2.09383632135605... = (9 + 4*sqrt(2))/7 = A156649 = R1 (Ratio 1). Lim. k -> Inf. a(2*k)/a(2*k-1) = 2.78361162489122432754 = (11 + 6*sqrt(2))/7 = R2 (Ratio 2); RG = R1*R2. a(2*k-1) = [ 2*[(3+2*Sqrt(2))^n - (3-2*Sqrt(2))^n] - [(3+2*Sqrt(2))^(n-1) - (3-2*Sqrt(2))^(n-1)] + [(3+2*Sqrt(2))^(n-2) - (3-2*Sqrt(2))^(n-2)] ] / (4*Sqrt(2)) a(2*k) = [ 5*[(3+2*Sqrt(2))^n - (3-2*Sqrt(2))^n] + [(3+2*Sqrt(2))^(n-1) - (3-2*Sqrt(2))^(n-1)] ] / (4*Sqrt(2)). a(n) = 6*a(n-2) - a(n-4). G.f.: x*(1+x)*(x^2+4*x+1) / ( (x^2+2*x-1)*(x^2-2*x-1) ). - R. J. Mathar, Jul 03 2011 a(n) = 6*a(n-2) - a(n-4) with a(1)=1, a(2)=5, a(3)=11, a(4)=31. - Sture Sjöstedt, Oct 08 2012 Bisection: a(2*k+1) = S(k, 6) + 5*S(k-1, 6), a(2*k) = 5*S(n-1, 6) + S(n-2, 6), with the Chebyshev polynomials S(n, x) (A049310) with S(-2, x) = -1, S(-1, x) = 0, evaluated at x = 6. S(n, 6) = A001109(n-1). See A054490 and A255236, and the given g.f.s. -  Wolfdieter Lang, Feb 26 2015 EXAMPLE n = 3: (A077447(3))^2 - 2*a(3)^2 = 16^2 - 2*11^2  = 14; a(3)^2 - 2*(2*A006452(3+1))^2 = 11^2 - 2*(2*4)^2 = -7. - Wolfdieter Lang, Feb 26 2015 MATHEMATICA LinearRecurrence[{0, 6, 0, -1}, {1, 5, 11, 31}, 50] (* Sture Sjöstedt, Oct 08 2012 *) CROSSREFS Cf. 2*(a(n))^2 + 14 = (A077447(n))^2. A006452, A155765, A054490, A255236. Sequence in context: A038580 A106088 A246010 * A023276 A074648 A236428 Adjacent sequences:  A077443 A077444 A077445 * A077447 A077448 A077449 KEYWORD nonn AUTHOR Gregory V. Richardson, Nov 09 2002 STATUS approved

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Last modified November 13 10:50 EST 2019. Contains 329093 sequences. (Running on oeis4.)