“…In parallel to the variational aspects of boundary-value problems on graphs studied here and on trees in [21], the work of Pokornyi and Pryadiev, and Pokornyi, Pryadiev and Al-Obeid, in [17] and [18], should be noted for the extension of Sturmian oscillation theory to second order operators on graphs. The idea of approximating the behaviour of eigenfunctions and eigenvalues for a boundary-value problem on a graph by the behaviour of associated problems on the individual edges, used here, was studied in the definite case in [2], [11] and [22].…”