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A216073 The list of the a(n)-values in the common solutions to  k+1=b^2 and 6*k+1=a^2. 1
1, 7, 17, 71, 169, 703, 1673, 6959, 16561, 68887, 163937, 681911, 1622809, 6750223, 16064153, 66820319, 159018721, 661452967, 1574123057, 6547709351, 15582211849, 64815640543, 154247995433, 641608696079, 1526897742481, 6351271320247, 15114729429377, 62871104506391 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The equations are equivalent to the Pell equation a^2-6*b^2=-5 with the 2 fundamental solutions (1;1) and (7;3) and the solution (5;2) for the unit form.

The associated b(n) are in A080806.

A181442(n)=(A080806(n)+1)/2.

A180483(n)=(A216073(n)+5)/2.

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (0,10,0,-1).

FORMULA

a(n) = 10*a(n-2) - a(n-4).

a(n) = a(n-1) + 10*a(n-2) - 10*a(n-3) - a(n-4) + a(n-5).

G.f. (x+7*x^2+7*x^3+x^4)/(1-10*x^2+x^4).

a(2*n+1) = ((1+r)*(5+2*r)^n+(1-r)*(5-2*r)^n)/2  where r=sqrt(6) and 0<=n.

a(2*n+2) = ((7+3*r)*(5+2*r)^n+(7-3*r)*(5-2*r)^n)/2  where r=sqrt(6) and 0<=n.

a(n) = -((5-2*r)^(1/4)*((2*r+5)^((-1)^n/4+n/2)*(-1)^n-r*(2*r+5)^((-1)^n/4+n/2))+(2*r+5)^(1/4)*((5-2*r)^((-1)^n/4+n/2)*(-1)^n+(5-2*r)^((-1)^n/4+n/2)*r))/(2*(5-2*r)^(1/4)*(2*r+5)^(1/4)) with r=sqrt(6) and 1<=n.- Alexander R. Povolotsky, Sep 01 2012

MAPLE

a(1)=1: a(2)=7: a(3)=17: a(4)=71:

for n from 5 to 20 do

  a(n)=10*a(n-2)-a(n-4):

  printf("%9d%20d\n", n, a(n)):

end do:

MATHEMATICA

LinearRecurrence[{0, 10, 0, -1}, {1, 7, 17, 71}, 50] (* G. C. Greubel, Feb 22 2017 *)

PROG

(PARI)

a(n) = if(n<1, 0, if(n<5, [1, 7, 17, 71][n], 10*a(n-2)-a(n-4) ) );

/* Joerg Arndt, Sep 03 2012 */

CROSSREFS

Sequence in context: A242907 A034054 A120876 * A086870 A107693 A217717

Adjacent sequences:  A216070 A216071 A216072 * A216074 A216075 A216076

KEYWORD

nonn,easy

AUTHOR

Paul Weisenhorn, Sep 01 2012

STATUS

approved

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Last modified July 29 05:00 EDT 2021. Contains 346340 sequences. (Running on oeis4.)