This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A045506 #21 Aug 29 2025 16:31:17 %S A045506 -1,2,3,5,6,8,9,11,12,14,15,17,18,20,21,23,24,26,27,29,30,32,33,35,36, %T A045506 38,39,41,42,44,45,47,48,50,51,53,54,56,57,59,60,62,63,65,66,68,69,71, %U A045506 72,74,75,77,78,80,81,83,84,86,87,89,90,92,93,95,96,98,99,101,102,104 %N A045506 Inscribe 2 spheres of curvature 2 inside sphere of curvature -1, continue to inscribe spheres where possible; sequence gives list of curvatures. %C A045506 A 3-dimensional Apollonian (or Soddy) problem. %C A045506 Appears to be congruent to 0 or 2 mod 3. %F A045506 Appears to coincide with a(n)=2+3*floor((n-1)/2)+(1+(-1)^n)/2-0^n, in which case the g.f. is (-1+3x+2x^2-x^3)/((1+x)(1-x)^2). - _Paul Barry_, Sep 11 2008 %Y A045506 Cf. A042944, A007494. %K A045506 sign,nice,changed %O A045506 0,2 %A A045506 _Brian Galebach_