How can I plot a Farey diagram?

I looked up the Farey sequence on Wikipedia, out of curiosity, because I had not heard of it before. The Farey sequence of order $n$ is "the sequence of completely reduced fractions between 0 and 1 which, when in lowest terms, have denominators less than or equal to $n$, arranged in order of increasing size".

On that basis, you can generate the sequence as follows, for instance:

ClearAll[farey]
farey[n_Integer] := (Divide @@@ Subsets[Range[n], {2}]) ~ Join ~ {0, 1} //DeleteDuplicates //Sort

So for instance:

farey[5]

{0, 1/5, 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 1}

I am not sure how these sequences are connected with the figure you showed though.

The curvilinear triangles which are characteristic for this type of plot are called hypocycloid curves. We can use the parametric equations on Wikipedia to plot these, like so:

x[a_, b_, t_] := (b - a) Cos[t] + a Cos[(b - a)/a t]
y[a_, b_, t_] := (b - a) Sin[t] - a Sin[(b - a)/a t]
hypocycloid[n_] := ParametricPlot[
  {x[1/n, 1, t], y[1/n, 1, t]},
  {t, 0, 2 Pi},
  PlotStyle -> {Thickness[0.002], Black}
  ]

Show[
 Graphics[Circle[{0, 0}, 1]],
 hypocycloid[2],
 hypocycloid[4],
 hypocycloid[8],
 hypocycloid[16],
 hypocycloid[32],
 hypocycloid[64],
 ImageSize -> 500
 ]

I've previously written about an application of hypocycloids here, and I showed how to visualize epicycloids here.

How to generate the labels is described here (also linked to by moo in a comment). I will simply provide the code.

mediant[{a_, b_}, {c_, d_}] := {a + c, b + d}
recursive[v1_, v2_, depth_] := If[
  depth > 2,
  mediant[v1, v2], {
   recursive[v1, mediant[v1, v2], depth + 1],
   mediant[v1, v2],
   recursive[mediant[v1, v2], v2, depth + 1]
   }]

computeLabels[v1_, v2_] := Module[{numbers},
  numbers = 
   Cases[recursive[v1, v2, 0], {_Integer, _Integer}, Infinity];
  StringTemplate["``/``"] @@@ numbers
  ]
computeLabelsNegative[v1_, v2_] := Module[{numbers},
  numbers = 
   Cases[recursive[v1, v2, 0], {_Integer, _Integer}, Infinity];
  StringTemplate["-`2`/`1`"] @@@ numbers
  ]

labels = Reverse@Join[
    {"1/0"},
    computeLabels[{1, 0}, {1, 1}],
    {"1/1"},
    computeLabels[{1, 1}, {0, 1}],
    {"0/1"},
    computeLabelsNegative[{1, 0}, {1, 1}],
    {"-1,1"},
    computeLabelsNegative[{1, 1}, {0, 1}]
    ];

coords = CirclePoints[{1.1, 186 Degree}, 64];

Show[
 Graphics[Circle[{0, 0}, 1]],
 hypocycloid[2],
 hypocycloid[4],
 hypocycloid[8],
 hypocycloid[16],
 hypocycloid[32],
 hypocycloid[64],
 Graphics@MapThread[Text, {labels, coords}],
 ImageSize -> 500
 ]

Using Graph with a bit of coding:

addPoint[{p : h_[a_,b_], q : h_[c_,d_]}, i_] :=
    With[{np = h[a + c, b + d]}, Sow[{p [UndirectedEdge] np, np [UndirectedEdge] q}]; Sow[{i, i}, "Depth"]; {p, np, q}]

addPoint[{p : h_[a_,b_], q : h_[-1][c_,d_]}, i_] :=
    With[{np = h[-1][a + c, b + d]}, Sow[{p [UndirectedEdge] np, np [UndirectedEdge] q}]; Sow[{i, i}, "Depth"]; {p, np, q}]

addPoint[{p : h_[-1][a_,b_], q : h_[c_,d_]}, i_] :=
    With[{np = h[-1][a + c, b + d]}, Sow[{p [UndirectedEdge] np, np [UndirectedEdge] q}]; Sow[{i, i}, "Depth"]; {p, np, q}]

addPoint[{p : h_[-1][a_,b_], q : h_[-1][c_,d_]}, i_] :=
    With[{np = h[-1][a + c, b + d]}, Sow[{p [UndirectedEdge] np, np [UndirectedEdge] q}]; Sow[{i, i}, "Depth"]; {p, np, q}]

fLabel[fr_, angle_] := 
    With[{tangle=ArcTan@@angle}, Placed[fLabel[fr], AngleVector[{1/2, 1/2}, {.7, #}] & /@{tangle, tangle+Pi}]]

fLabel[h_[a_, b_]] := ToString[a] ~~ "/" ~~ ToString[b]
fLabel[h_[-1][a_, b_]] := "-" ~~ ToString[a] ~~ "/" ~~ ToString[b]

FareyDiagram[n_Integer, d_Integer: 1, opts___?OptionQ] :=
    Block[{fr, top, bottom, stedges, toppart, bottompart, vert, edges, coords, labels, labpos, cfunc, i, edgestyle, dstyle, nopts},
        cfunc = ColorFunction /. Flatten[{opts}] /. ColorFunction -> Automatic;
        nopts = FilterRules[Flatten[{opts}], Options[Graph]];
        top = {fr[0,1], fr[1,1], fr[1,0]};
        bottom = {fr[1,0], fr[-1][1,1], fr[0,1]};
        stedges = UndirectedEdge@@@Join[Partition[top, 2, 1], Partition[bottom, 2, 1], {{fr[0, 1],fr[1, 0]}}];
        i = 0;toppart = Reap[Nest[(i++; Split[Flatten[addPoint[#, i] & /@ Partition[#, 2, 1],1]][[All,1]])&, top, n]];
        i = 0;bottompart = Reap[Nest[(i++; Split[Flatten[addPoint[#, i] & /@ Partition[#,2,1],1]][[All,1]])&,bottom, n]];
        vert = Join[toppart[[1]], bottompart[[1,  2;;-2]]];
        edges = Flatten[{stedges, toppart[[2, 1]], bottompart[[2, 1]]}];
        coords = CirclePoints[{1,0},Length[vert]];
        labpos = Range[1, Length[vert], 2 ^ (d - 1)];
        labels = Thread[vert[[labpos]]->fLabel@@@Transpose[{vert,coords}][[labpos]]];
        edgestyle = Black;
        dstyle = Black;
        If[cfunc =!= Automatic,
            edgestyle = Flatten[{Table[0, Length[stedges]], toppart[[2, 2]], bottompart[[2, 2]]}];
            edgestyle = edgestyle / Max[edgestyle];
            edgestyle = Thread[edges -> Flatten[cfunc[1 - #] & /@ edgestyle]];
            dstyle = cfunc[1]
         ];
        Graph[vert, edges, nopts, VertexCoordinates->CirclePoints[{1,0},Length[vert]], VertexLabels->labels, 
           EdgeShapeFunction->(BSplineCurve[{#1[[1]],{0,0},#1[[2]]}, SplineWeights->{2,EuclideanDistance@@#,2}]&), 
           PerformanceGoal->"Speed", Epilog->{dstyle, Circle[]}, VertexShapeFunction -> "Point", EdgeStyle -> edgestyle, VertexStyle -> dstyle]
    ]

Example:

FareyDiagram[4]

FareyDiagram[6, 4, ColorFunction -> Hue, 
 VertexLabelStyle -> Darker[Red]]

grupo[n_] := Show[{Graphics[{Thin, Red, 
Circle[{0, 0}, 1, {0, Pi/2}]}]}, {Graphics[{Thin, 
Map[{BSplineCurve[{#1[[1]], {0, 0}, #1[[2]]}, 
SplineWeights -> {2, EuclideanDistance @@ 
#,2}]}&, 
Partition[ReIm[Exp[Pi/2 I #]] & /@
FareySequence[n], 2, 1]]}]}, {Map[Graphics[{Blue, 
Point[{ReIm[Exp[Pi/2 I #]]}]}] &, 
FareySequence[n]]}, PlotRange -> All]

Show[Table[grupo[n], {n, 2, 7}]]

While reading an excellent post on chord diagrams, I realized that this type of figure is a Poincaré hyperbolic disk. The lines are Poincaré arcs. There is MathWorld article with code here.

So, copying the definitions of PoincareArc and PoincareDisk from MathWorld, we can draw the figure like this:

PoincareArc[l0_List] := 
 Module[{l = Sort[l0], dt, t, t1, t2, r, R, c},
  dt = Abs[l[[1]] - l[[2]]];
  t = Plus @@ l/2;
  If[
   dt == Pi,
   Line[{
     {Cos[l[[1]]], Sin[l[[1]]]},
     {Cos[l[[2]]], Sin[l[[2]]]}
     }], c = {Cos[t], Sin[t]};
   r = Tan[dt/2];
   R = Sec[dt/2];
   t1 = ArcTan @@ ({Cos[l[[2]]], Sin[l[[2]]]} - R c);
   t2 = ArcTan @@ ({Cos[l[[1]]], Sin[l[[1]]]} - R c);
   If[t2 < t1, t2 += 2 Pi];
   Circle[R c, r, {t1, t2}]]]

PoincareDisk[l_List] := 
 Module[{i}, {{Thickness[.001], Circle[{0, 0}, 1]}, 
   Table[PoincareArc[l[[i]]], {i, Length[l]}]}]

angles[n_] := Partition[Table[th, {th, 0, 2 Pi, 2 Pi/n}], 2, 1]

Graphics[{
  PoincareDisk@angles[2],
  PoincareDisk@angles[4],
  PoincareDisk@angles[8],
  PoincareDisk@angles[16],
  PoincareDisk@angles[32],
  PoincareDisk@angles[64]
  }]

An alternative way to use Graph: Using CycleGraphs with 2^k vertices and the internal edge shape function GraphComputation`GraphChartDump`pEdge:

Quiet[GraphComputation`GraphPropertyChart[]];
eSF = GraphComputation`GraphChartDump`pEdge[#2[[1]], blah, blah, ##] &;

Show[CycleGraph[2^#, VertexSize -> 0, EdgeShapeFunction -> eSF, 
   EdgeStyle -> Black] & /@ Range[5], Epilog -> Circle[]]

Alternatively, use a function to define the edge list to be used in a single Graph:

ClearAll[vPairs]
vPairs[k_Integer] := DeleteDuplicates[Join @@ 
  (Partition[Append[#, 1], 2, 1] & /@ Range[1, 2^k, 2^Range[0, k-1]])]

Graph[Range[2^k], vPairs[k], 
  EdgeStyle->Black,
  VertexShape -> Graphics[{}], 
  Epilog -> Circle[],
  EdgeShapeFunction -> eSF,
  GraphLayout -> "CircularEmbedding",
  ImageSize -> 300]

same picture

With colored edges:

Row[(colors = RotateRight @ ColorData[97, "ColorList"];
  Show[CycleGraph[2^#, VertexSize -> 0, EdgeShapeFunction -> eSF, 
         EdgeStyle -> Directive[Thick, First[colors = RotateLeft[colors]]]] & /@ Range[#], 
    Epilog -> Circle[], ImageSize -> 300]) & /@ Range[2, 5]]