Changes

<p>'''Definition 1.2.'''  A ''graph'' (usually called a ''directed graph'') consists of two classes:  the class of ''arrows'' (or ''oriented edges'') and the class of ''objects'' (usually called ''nodes'' or ''vertices'') and two mappings from the class of arrows to the class of objects, called ''source'' and ''target'' (often also ''domain'' and ''codomain'').</p>

<p>'''Definition 1.2.'''  A ''graph'' (usually called a ''directed graph'') consists of two classes:  the class of ''arrows'' (or ''oriented edges'') and the class of ''objects'' (usually called ''nodes'' or ''vertices'') and two mappings from the class of arrows to the class of objects, called ''source'' and ''target'' (often also ''domain'' and ''codomain'').</p>

<br><pre>

<center><pre>

 

o--------------o      source      o--------------o

o--------------o      source      o--------------o

|              | ----------------> |              |

|              | ----------------> |              |

|              | ----------------> |              |

|              | ----------------> |              |

o--------------o      target      o--------------o

o--------------o      target      o--------------o

</pre><br>

 

</pre></center>

<p>One writes <math>^{\backprime\backprime} f : A \to B \, ^{\prime\prime}</math> for <math>^{\backprime\backprime} \operatorname{source}\ f = A ~\operatorname{and}~ \operatorname{target}\ f = B \, ^{\prime\prime}.</math>  A graph is said to be ''small'' if the classes of objects and arrows are sets.</p>

<p>One writes <math>^{\backprime\backprime} f : A \to B \, ^{\prime\prime}</math> for <math>^{\backprime\backprime} \operatorname{source}\ f = A ~\operatorname{and}~ \operatorname{target}\ f = B \, ^{\prime\prime}.</math>  A graph is said to be ''small'' if the classes of objects and arrows are sets.</p>