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A074074 The numbers D in the set {D :=(2n+1)^2-4m^2, 1<=m<=n} that generate the smallest solution x to x^2 - D*y^2 = 1. 3
5, 21, 33, 17, 105, 105, 189, 33, 105, 405, 333, 141, 473, 57, 817, 189, 325, 885, 77, 1425, 1173, 1925, 1425, 2301, 101, 105, 1425, 333, 777, 1785, 2525, 381, 1785, 405, 141, 3393, 2261, 3813, 6045, 2717, 4389, 3129, 2093, 6765, 885, 5513, 189, 6045, 197, 8085 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Given a discriminant D, the Pell equation x^2-D*y^2=1 has a minimum solution x

as tabulated in A033313. We start with a set of candidates D of the form

(2*n+1)^2-(2*m)^2, obviously all odd, where m runs through the integers from 1 to n.

Whichever D out of this set generates the smallest x in A033313, defines a(n)=D.

LINKS

Ray Chandler, Table of n, a(n) for n = 1..499

EXAMPLE

For n=4, the candidates are D=77 (m=1, index 69 in A000037), D=65 (m=2, index 57 in A000037),

D=45 (m=3, index 39 in A000037) and D=17 (m=4, index 13 in A000037),

which produce x = 351, x=129, x=161 and x=33 in that order (apply the offset in A033313

while converting indices from A000037 to find the x). Because 33 is the smallest

of these four x, we select the associated D=17 as a(n=4).

MAPLE

A033313 := proc(Dcap) local c, i, fr, nu, de ; if issqr(Dcap) then -1; else c := numtheory[cfrac](sqrt(Dcap)) ; for i from 1 do try fr := numtheory[nthconver](c, i) ; nu := numer(fr) ; de := denom(fr) ; if nu^2-Dcap*de^2=1 then RETURN(nu) ; fi; catch: RETURN(-1) ; end try; od: fi: end:

A074074 := proc(n) local Dmin, xmin, Dcap ; Dmin := -1 ; xmin := -1; for m from 1 to n do Dcap := (2*n+1+2*m)*(2*n+1-2*m) ; x := A033313(Dcap) ; if xmin = -1 or (x >0 and x<xmin ) then Dmin := Dcap ; xmin := x ; fi; od: Dmin ; end:

seq(A074074(n), n=1..50) ; # R. J. Mathar, Sep 21 2009

CROSSREFS

Cf. A074075 (associated m values), A074076.

Sequence in context: A176300 A042319 A186304 * A006309 A273577 A145026

Adjacent sequences:  A074071 A074072 A074073 * A074075 A074076 A074077

KEYWORD

nonn

AUTHOR

Lekraj Beedassy, Aug 28 2002

EXTENSIONS

Definition clarified, sequence extended beyond a(7) - R. J. Mathar, Sep 21 2009

STATUS

approved

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Last modified March 25 19:41 EDT 2017. Contains 284082 sequences.