The line digraph of a directed graph is the directed graph whose vertex
set corresponds to the arc set of and having an arc directed from
an arc
to an arc
if, in ,
the head of
meets the tail of
(Harary and Palmer 1973, p. 264; Gross and Yellen 2006, p. 265).
Gross, J. T. and Yellen, J. Graph Theory and Its Applications, 2nd ed. Boca Raton, FL: CRC Press, p. 265,
2006.Harary, F. and Palmer, E. M. "A Survey of Graphical Enumeration
Problems." In A Survey of Combinatorial Theory (Ed. J. N. Srivastava).
Amsterdam, Netherlands: North-Holland, pp. 259-275, 1973.
is the directed graph 
whose vertex
set corresponds to the arc set of 
and having an arc directed from
an arc 
to an arc 
if, in 
,
the head of 
meets the tail of 
(Harary and Palmer 1973, p. 264; Gross and Yellen 2006, p. 265).
