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 A106521 Numbers a(n) such that Sum[k=0..10, (a(n)+k)^2 ] is square. 7
 18, 38, 456, 854, 9192, 17132, 183474, 341876, 3660378, 6820478, 73024176, 136067774, 1456823232, 2714535092, 29063440554, 54154634156, 579811987938, 1080378148118, 11567176318296, 21553408328294 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Equivalently, 11*a(n)^2 + 110*a(n) + 385 is square. 11*((m+5)^2+10) is a square iff the second factor is divisible by 11 and the quotient is a square, i.e. iff m = 11 k - 4 or m = 11 k - 6 and 11 k^2 +/- 2 k + 1 is a square. Thus a(n)=(7,5,5,7,7,5,5,7,...) (mod 11), repeating with period 4 and the values are obtained by solving these Pell-type equations, e.g. using http://www.alpertron.com.ar/QUAD.HTM. The corresponding recurrence equations (see PARI code) should make it possible to prove the conjectured g.f. - M. F. Hasler, Jan 27 2008 All sequences of this type (i.e. sequences with fixed offset k, and a discernible pattern: k=0...10 for this sequence, k=0..1 for A001652) can be continued using a formula such as x(n) = a*x(n-p) - x(n-2p) + b, where a and b are various constants, and p is the period of the series. Alternatively 'p' can be considered the number of concurrent series. - Daniel Mondot, Aug 05 2016 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..500 Index entries for linear recurrences with constant coefficients, signature (1,20,-20,-1,1). FORMULA G.f.: 2*x*(9+10*x+29*x^2-x^3-2*x^4)/(1-x)/(1-20*x^2+x^4). - Vladeta Jovovic, May 31 2005 a(1)=18, a(2)=38, a(3)=456, a(4)=854, a(5)=9192; thereafter a(n)=a(n-1)+20*a(n-2)- 20*a(n-3)-a(n-4)+a(n-5). - Harvey P. Dale, May 07 2011 a(n) = A198949(n+1)-5. - Bruno Berselli, Feb 12 2012 a(1)=18, a(2)=38, a(3)=456, a(4)=854; thereafter a(n) = 20*a(n-2) - a(n-4) + 90. - Daniel Mondot, Aug 05 2016 EXAMPLE Since 18^2 + 19^2 + ... + 28^2 = 5929 = 77^2, 18 is in the sequence. - Michael B. Porter, Aug 07 2016 MATHEMATICA LinearRecurrence[{1, 20, -20, -1, 1}, {18, 38, 456, 854, 9192}, 30] (* Harvey P. Dale, May 07 2011 *) PROG (PARI) A106521(n)={local(xy=[ -4-2*(n%2); 11], PQRS=[10, 3; 33, 10], KL=[45; 165]); until(0>=n-=2, xy=PQRS*xy+KL); xy} \\ M. F. Hasler, Jan 27 2008 CROSSREFS Cf. A001032, A094196. Sequence in context: A084585 A132761 A079862 * A070686 A043118 A039295 Adjacent sequences:  A106518 A106519 A106520 * A106522 A106523 A106524 KEYWORD nonn,easy AUTHOR Ralf Stephan, May 30 2005 EXTENSIONS Edited and extended by M. F. Hasler, Jan 27 2008 G.f. adapted to the offset by Bruno Berselli, May 16 2011 STATUS approved

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Last modified March 28 17:51 EDT 2020. Contains 333103 sequences. (Running on oeis4.)