- مبلغ: ۸۶,۰۰۰ تومان
- مبلغ: ۹۱,۰۰۰ تومان
Let G be a digraph and LG be its line digraph. Levine gave a formula that relates the number of rooted spanning trees of LG and that of G, with the restriction that G has no sources. In this note, we show that this restriction can be removed, thus his formula holds for all digraphs.