Graph Union
The union 
of graphs 
and 
with disjoint
point sets 
and 
and edge sets
and 
is the graph
with 
and 
(Harary 1994, p. 21; Gross and Yellen 2006, p. 85). This operation is sometimes
also known explicitly as the graph disjoint union.
Graph unions can be computed in the Wolfram Language using GraphDisjointUnion[g1, g2, ...]. (Note that the Wolfram Language function GraphUnion[g1, g2] performs a different operation from the usual graph union.)
The graph union of 
copies of a graph 
is commonly denoted
(Harary 1990, p. 21).
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