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 A322709 a(0)=0, a(1)=7 and a(n) = 30*a(n-1) - a(n-2) + 14 for n > 1. 2
 0, 7, 224, 6727, 201600, 6041287, 181037024, 5425069447, 162571046400, 4871706322567, 145988618630624, 4374786852596167, 131097616959254400, 3928553721925035847, 117725514040791821024, 3527836867501829594887, 105717380511014096025600, 3167993578462921051173127 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Also numbers k such that 7*A000217(k) is a square. - Metin Sariyar, Nov 16 2019 LINKS Colin Barker, Table of n, a(n) for n = 0..650 Index entries for linear recurrences with constant coefficients, signature (31,-31,1). FORMULA sqrt(a(n)+1) + sqrt(a(n)) = (sqrt(8) + sqrt(7))^n. sqrt(a(n)+1) - sqrt(a(n)) = (sqrt(8) - sqrt(7))^n. a(n) = 31*a(n-1) - 31*a(n-2) + a(n-3) for n > 2. From Colin Barker, Dec 25 2018: (Start) G.f.: 7*x*(1 + x) / ((1 - x)*(1 - 30*x + x^2)). a(n) = ((15+4*sqrt(14))^(-n) * (-1+(15+4*sqrt(14))^n)^2) / 4. (End) E.g.f.: (1/4)*(-2*exp(x) + exp((15-4*sqrt(14))*x) + exp((15+4*sqrt(14))*x)). - Stefano Spezia, Nov 16 2019 EXAMPLE (sqrt(8) + sqrt(7))^2 = 15 + 2*sqrt(56) = sqrt(225) + sqrt(224). So a(2) = 224. MATHEMATICA LinearRecurrence[{31, -31, 1}, {0, 7, 224}, 18] (* Metin Sariyar, Nov 23 2019 *) PROG (PARI) concat(0, Vec(7*x*(1 + x) / ((1 - x)*(1 - 30*x + x^2)) + O(x^20))) \\ Colin Barker, Dec 25 2018 (MAGMA) a:=[0, 7]; [n le 2 select a[n] else 30*Self(n-1)-Self(n-2)+14: n in [1..18]]; // Marius A. Burtea, Nov 16 2019 (MAGMA) R:=PowerSeriesRing(Integers(), 18); [0] cat Coefficients(R!(7*x*(1 + x) / ((1 - x)*(1-30*x + x^2))));  // Marius A. Burtea, Nov 16 2019 CROSSREFS Row 7 of A322699. Cf. A188932 (sqrt(7)+sqrt(8)). Sequence in context: A009488 A302059 A138247 * A015506 A210099 A193503 Adjacent sequences:  A322706 A322707 A322708 * A322710 A322711 A322712 KEYWORD nonn,easy AUTHOR Seiichi Manyama, Dec 24 2018 STATUS approved

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Last modified March 31 19:37 EDT 2020. Contains 333151 sequences. (Running on oeis4.)