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A298101 Expansion of x*(1 + x)/((1 - x)*(1 - 322*x + x^2)). 2
0, 1, 324, 104329, 33593616, 10817040025, 3483053294436, 1121532343768369, 361129931640120384, 116282716455774995281, 37442673568827908360100, 12056424606446130716956921, 3882131280602085262951768464, 1250034215929265008539752488489 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

16*k*a(n) provides infinitely many x-values solutions (x,y) of x*(5*x + k) = y^2.

This follows from the fact that 5*16*a(n) + 1 is a perfect square: more precisely, 80*a(n) + 1 = A023039(n)^2.

This is a divisibility sequence, that is a(n) divides a(m) if n divides m. It is the case P1 = 324, P2 = 644, Q = 1 of the 3 parameter family of 4th-order linear divisibility sequences found by Williams and Guy. - Peter Bala, Jan 19 2018

LINKS

Colin Barker, Table of n, a(n) for n = 0..300

H. C. Williams and R. K. Guy, Some fourth-order linear divisibility sequences, Intl. J. Number Theory 7 (5) (2011) 1255-1277.

Index entries for linear recurrences with constant coefficients, signature (323,-323,1).

FORMULA

G.f.: x*(1 + x)/((1 - x)*(1 - 322*x + x^2)).

a(n) = a(-n) = ((2 + sqrt(5))^(4*n) + (2 - sqrt(5))^(4*n) - 2)/320.

a(n) = A225786(n)/48. This is the case k=3 of the first comment. Example: for n = 2, 16*3*a(2) = A225786(2) = 15552 and 15552*(5*15552+3) = 34776^2.

a(n) = A049660(n)^2.

a(n)*(80*a(n) + 1) = 81*A253368(n)^2 for n>0.

a(n)*a(n-2) = (a(n-1) - 1)^2.

a(n) = 322*a(n-1) - a(n-2) + 2.

a(n) = 323*a(n-1) - 323*a(n-2) + a(n-3). - Iain Fox, Jan 12 2018

MAPLE

P:=proc(n) trunc(evalf(((2+sqrt(5))^(4*n)+(2-sqrt(5))^(4*n)-2)/320, 1000));

end: seq(P(i), i=0..13); # Paolo P. Lava, Jan 18 2018

MATHEMATICA

CoefficientList[x (1 + x)/((1 - x) (1 - 322 x + x^2)) + O[x]^20, x]

PROG

(Sage) gf = x*(1+x)/((1-x)*(1-322*x+x^2)); print taylor(gf, x, 0, 20).list()

(Maxima) makelist(coeff(taylor(x*(1+x)/((1-x)*(1-322*x+x^2)), x, 0, n), x, n), n, 0, 20);

(PARI) first(n) = Vec(x*(1 + x)/((1 - x)*(1 - 322*x + x^2)) + O(x^n), -n) \\ Iain Fox, Jan 12 2018

CROSSREFS

Cf. A023039, A049660, A225786, A253368, A298271.

Sequence in context: A088216 A121001 A203029 * A209044 A209016 A132644

Adjacent sequences:  A298098 A298099 A298100 * A298102 A298103 A298104

KEYWORD

nonn,easy

AUTHOR

Bruno Berselli, Jan 12 2018

STATUS

approved

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Last modified January 26 11:04 EST 2020. Contains 331279 sequences. (Running on oeis4.)