A signed graph (G,Σ) is a G together with set Σ⊆E(G) of negative edges. circuit positive if the product signs its edges positive. balanced all circuits are The frustration index l(G,Σ) minimum cardinality E⊆E(G) such that (G−E,Σ−E) balanced, and k-critical l(G,Σ)=k l(G−e,Σ−e)<k, for every e∈E(G). We study decomposition subdivision critical graphs completely determine t-critical graphs, t≤2. Cri...

Let σ = {σi|i ∈ I } be a partition of the set all primes ℙ and G finite group. A ℋ subgroups is said to complete Hall σ-set if every member ≠1 σi-subgroup for some i contains exactly one such that σi ⌒ π(G) ≠ ∅. group σ-primary it σi-group i. subgroup be: σ-permutable in possesses AH x H xA G; σ-subnormal there chain A0 ≤ A1 … At either Ai − 1 ⊴ or /(Ai 1)Ai 1, …, t;