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.

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

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