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A256694 The radius of the largest of four circles with different integer radii arranged so that each circle is tangent externally to the other three circles. 2
69, 70, 132, 138, 140, 153, 198, 207, 210, 216, 238, 252, 264, 264, 264, 270, 276, 280, 285, 290, 306, 345, 350, 390, 396, 396, 414, 420, 429, 432, 459, 476, 483, 490, 504, 504, 520, 528, 528, 528, 539, 540, 552, 560, 567, 570, 580, 594, 595, 612, 621, 630 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..52.

Kival Ngaokrajang, Illustration of the first three solutions

Eric Weisstein's World of Mathematics, Soddy Circles.

PROG

(PARI)

soddy(k) = {

  s=[];

  for(a=1, k,

    for(b=1, a-1,

      for(c=1, b-1,

        if(issquare(a*b*c*(a+b+c), &t),

          if(a*b*c % (a*b+a*c+b*c+2*t) == 0,

            s=concat(s, a)

          )

        )

      )

    )

  );

  s

}

soddy(500)

CROSSREFS

See also the many sequences arising from Apollonian circle packing: A135849, A137246, A154636, etc.

Also the sequences related to Soddy's circle packings: A046159, A046160, A062536, etc.

Sequence in context: A265189 A345486 A253430 * A166067 A253437 A325386

Adjacent sequences:  A256691 A256692 A256693 * A256695 A256696 A256697

KEYWORD

nonn

AUTHOR

Colin Barker, Apr 08 2015

STATUS

approved

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Last modified August 10 06:08 EDT 2022. Contains 356029 sequences. (Running on oeis4.)