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 A038761 a(n) = 6*a(n-1)-a(n-2), n >= 2, a(0)=1, a(1)=9. 29
 1, 9, 53, 309, 1801, 10497, 61181, 356589, 2078353, 12113529, 70602821, 411503397, 2398417561, 13979001969, 81475594253, 474874563549, 2767771787041, 16131756158697, 94022765165141, 548004834832149 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Bisection of A048654. - Lambert Klasen (lambert.klasen(AT)gmx.de), Nov 24 2004 This gives part of the (increasingly sorted) positive solutions y to the Pell equation x^2 - 2*y^2 = +7. For the x solutions see A038762. For the other part of solutions see A101386 and A253811. - Wolfdieter Lang, Feb 05 2015 REFERENCES A. H. Beiler, Recreations in the Theory of Numbers, Dover, N.Y., 1964, pp. 122-125, 194-196. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..400 M. J. DeLeon, Pell's Equation and Pell Number Triples, Fib. Quart., 14(1976), pp. 456-460. Tanya Khovanova, Recursive Sequences Index entries for linear recurrences with constant coefficients, signature (6,-1). FORMULA a(n) = {9*([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)). A038761 = sqrt{2*(A038762)^2-14}/2. For n>1, a(n)-4a(n-1)=A001541(n)-A001542(n-2); e.g. 309-4*53=97=99-2. - Charlie Marion, Nov 12 2003 For n>0, a(n)=A046090(n)+A001653(n)+A001652(n-1)=A055997(n+1)+A001652(n-1); e.g., 309=120+169+20. - Charlie Marion, Oct 11 2006 G.f.: (1+3*x)/(1-6*x+x^2). - Philippe Deléham, Nov 03 2008 a(n) = third binomial transform of 1,6,8,48,64,384. - Al Hakanson (hawkuu(AT)gmail.com), Aug 15 2009 a(n)^2 + 2^2 = A124124(2*n+1)^2 + (A124124(2*n+1)+1)^2. - Hermann Stamm-Wilbrandt, Aug 31 2014 a(n) = irrational part of z(n) = (3 + sqrt(2))*(3 + 2*sqrt(2))^n, n >= 0. z(n) gives only part of the general positive solutions to the Pell equation x^2 - 2*y^2 = 7). See the Nagell reference in A038762 on how to find z(n), and a comment above. - Wolfdieter Lang, Feb 05 2015 a(n) = S(n, 6) + 3*S(n-1, 6), n >= 0, with the Chebyshev S-polynomials evaluated at x=6. See S(n-1, 6) = A001109(n). - Wolfdieter Lang, Mar 30 2015 EXAMPLE A038762(3)^2 - 2*a(4)^2 = 2547^2 - 2*1801^2 = +7. - Wolfdieter Lang, Feb 05 2015 MAPLE a[0]:=1: a[1]:=9: for n from 2 to 26 do a[n]:=6*a[n-1]-a[n-2] od: seq(a[n], n=0..19); # Zerinvary Lajos, Jul 26 2006 MATHEMATICA LinearRecurrence[{6, -1}, {1, 9}, 40] (* Vincenzo Librandi, Nov 16 2011 *) PROG (MAGMA) I:=[1, 9]; [n le 2 select I[n] else 6*Self(n-1)-Self(n-2): n in [1..40]]; // Vincenzo Librandi, Nov 16 2011 (PARI) a(n)=([0, 1; -1, 6]^n*[1; 9])[1, 1] \\ Charles R Greathouse IV, Apr 08 2016 CROSSREFS Cf. A001541, A001542, A001652, A001653, A038762, A046090, A048654, A055997, A124124. Sequence in context: A122588 A277999 A295203 * A003698 A001688 A144040 Adjacent sequences:  A038758 A038759 A038760 * A038762 A038763 A038764 KEYWORD easy,nonn AUTHOR Barry E. Williams, May 02 2000 EXTENSIONS Edited: Replaced the unspecific Pell comment. Moved a formula from the comment section to the formula section. - Wolfdieter Lang, Feb 05 2015 STATUS approved

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