Eigenvalues of simple graphs

g1=Graph[{
1\[UndirectedEdge]2,2\[UndirectedEdge]3,
1\[UndirectedEdge]3
}]

g2=Graph[{
1\[UndirectedEdge]2,2\[UndirectedEdge]3
}]

a1=Normal[AdjacencyMatrix[g1]];
MatrixForm[a1]

Eigenvalues[a1]

a2=Normal[AdjacencyMatrix[g2]];
MatrixForm[a2]

Eigenvalues[a2]

g3=Graph[{
1\[UndirectedEdge]2,2\[UndirectedEdge]3,
1\[UndirectedEdge]3,1\[UndirectedEdge]4
}]

a3=Normal[AdjacencyMatrix[g3]];
MatrixForm[a3]
Eigenvalues[a3]

N[%]