Etydhv

This picture represented a graph which is connected to every node. How to use tikz to draw this kind of graph?

This is not your graph, but my recommended variant of it.

documentclass[tikz]standalone
begindocument
begintikzpicture
beginscope[every node/.style=circle,minimum size=.75cm,draw]
 node (a) at (0,2) Goal;
 node (c2) at (0,0) $c_2$;
 node (c3) at (2,0) $c_3$;
 node (c1) at (-2,0) $c_1$;
 node (a1) at (-1,-2) $a_1$;
 node (a2) at (1,-2) $a_2$;
endscope
draw (a)--(c1)--(a1)--(c2)--(a2)--(c3)--(a)--(c2) (c1)--(a2) (c3)--(a1);
endtikzpicture
enddocument

Some notes:

• The size of the nodes should not be too big. Hence I use minimum size=.75cm.


• The nodes should be in circle shapes.


• 
  

  You should not have all paths from a specific nodes connected. For instance, this is bad, in my opinion.

  
  
  

  

  
  
  

  It is even worse if you want to add arrows.

  
  



• The nodes should have a good alignment. In this graph, they should be horizontally centered.



• Put node label in math mode, if they are something like a, b, m₁, n₂, etc.

Based on the pencil drawing, here is a sample code, which you can improve according to your needs:

documentclassarticle
usepackagetikZ
usetikzlibrarypositioning

tikzsetsquarenode/.style = 
 shape = rectangle,
 draw = black,
 minimum height = 2cm,
 minimum width = 2cm


begindocument
begintikzpicture[node distance=2cm]
 node (a) at (0,0) [squarenode] Goal;
 node[below=of a] (c2) [squarenode] C2;
 node[left=of c2] (c1) [squarenode] C1;
 node[right=of c2] (c3) [squarenode] C3;
 node[below=of c1] (a1) [squarenode] A1;
 node[below=of c2] (a2) [squarenode] A2;

 draw (a.south) to (c1.north);
 draw (a.south) to (c2.north);
 draw (a.south) to (c3.north);
 draw (c1.south) to (a1.north);
 draw (c1.south) to (a2.north west);
 draw (c2.south) to (a2.north);
 draw (c2.south) to (a1.north east);
 draw (c3.south) to (a1.north);
 draw (c3.south) to (a2.north);
endtikzpicture
enddocument 

To understand the anchor points used, refer the diagram and code below:

documentclassarticle
usepackagetikZ
usetikzlibrarypositioning

tikzsetdot/.style = 
 shape = circle,
 draw = black,
 fill = black,
 minimum size = 0.2cm


begindocument
begintikzpicture[node distance=2cm]
 node (a) at (0,0) [squarenode] ;
 node[label=a.center] at (a.center) [dot] ; 
 node[label=a.north] at (a.north) [dot] ;
 node[label=a.south] at (a.south) [dot] ;
 node[label=a.east] at (a.east) [dot] ;
 node[label=a.west] at (a.west) [dot] ;
 node[label=a.north east] at (a.north east) [dot] ;
 node[label=a.north west] at (a.north west) [dot] ;
 node[label=a.north east] at (a.south east) [dot] ;
 node[label=a.north east] at (a.south west) [dot] ;
endtikzpicture
enddocument 

I think this is a job for a TikZ matrix.

With <matrix-name>-<row-number>-<column-number> you can refer to matrix cells as nodes.

Here you find two versions of the graph, if you want the nodes to be squared, leave out the circle option:

documentclassbook

usepackageamsmath
usepackagetikz
usetikzlibrarymatrix
tikzset
 mymatrix/.style=
 matrix of math nodes,
 nodes=draw, circle,
 row sep=10ex,
 

begindocument
begintikzpicture
 matrix[
 mymatrix,
 column sep=3em
 ](mymatr)
 &textGoal\
 C_1 & C_2& C_3\
 A_1 & A_2\
 ;
 foreach ind in 1,2,3 
 draw (mymatr-1-2) -- (mymatr-2-ind);
 draw (mymatr-3-1) -- (mymatr-2-ind);
 draw (mymatr-3-2) -- (mymatr-2-ind);
 
endtikzpicture

begintikzpicture
 matrix[
 mymatrix,
 column sep=1.5em
 ](mymatr)
 &&textGoal\
 C_1 && C_2&& C_3\
 &A_1 && A_2\
 ;
 foreach ind in 1,3,5 
 draw (mymatr-1-2) -- (mymatr-2-ind);
 draw (mymatr-3-2) -- (mymatr-2-ind);
 draw (mymatr-3-4) -- (mymatr-2-ind);
 
endtikzpicture
enddocument