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 A198947 x values in the solution to 11*x^2 - 10 = y^2. 5
 1, 7, 13, 139, 259, 2773, 5167, 55321, 103081, 1103647, 2056453, 22017619, 41025979, 439248733, 818463127, 8762957041, 16328236561, 174819892087, 325746268093, 3487634884699, 6498597125299, 69577877801893, 129646196237887, 1388069921153161, 2586425327632441 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS When are n and 11*n+1 perfect squares? This problem gives rise to the Diophantine equation 11*x^2 - 10 = y^2. Positive values of x (or y) satisfying x^2 - 20xy + y^2 + 90 = 0. - Colin Barker, Feb 18 2014 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (0, 20, 0, -1). FORMULA a(n+4) = 20*a(n+2) - a(n) with a(1)=1, a(2)=7, a(3)=13, a(4)=139. From Bruno Berselli, Nov 06 2011:  (Start) G.f.: x*(1-x)*(1+8*x+x^2)/(1-20*x^2+x^4). a(n) = ((11+(-1)^n*t)*(10-3*t)^floor(n/2)+(11-(-1)^n*t)*(10+3*t)^floor(n/2))/22 with t=sqrt(11).  (End). MATHEMATICA LinearRecurrence[{0, 20, 0, -1}, {1, 7, 13, 139}, 30] (* Vincenzo Librandi, Feb 06 2012 *) PROG From Bruno Berselli, Nov 07 2011:  (Start) (PARI) v=vector(25); v[1]=1; v[2]=7; v[3]=13; v[4]=139; for(i=5, #v, v[i]=20*v[i-2]-v[i-4]); v (MAGMA) m:=26; R:=PowerSeriesRing(Integers(), m); Coefficients(R!(x*(1-x)*(1+8*x+x^2)/(1-20*x^2+x^4))); (Maxima) makelist(expand(((11+(-1)^n*sqrt(11))*(10-3*sqrt(11))^floor(n/2)+(11-(-1)^n*sqrt(11))*(10+3*sqrt(11))^floor(n/2))/22), n, 1, 25); (End) CROSSREFS Cf. A198949, A221762. Sequence in context: A192894 A241280 A219703 * A342717 A050946 A178956 Adjacent sequences:  A198944 A198945 A198946 * A198948 A198949 A198950 KEYWORD nonn,easy AUTHOR Sture Sjöstedt, Oct 31 2011 EXTENSIONS Terms a(1)-a(7) confirmed, a(8)-a(15) added by John W. Layman, Nov 04 2011 a(16)-a(25) from Bruno Berselli, Nov 06 2011 STATUS approved

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Last modified June 24 00:04 EDT 2021. Contains 345403 sequences. (Running on oeis4.)