A group G admitting a finite subnormal series 1 = G_0 ◁ G_1 ◁ … ◁ G_n = G with each G_{i+1}/G_i abelian. Galois: an equation is solvable by radicals iff its Galois group is solvable.
A group G admitting a finite subnormal series 1 = G_0 ◁ G_1 ◁ … ◁ G_n = G with each G_{i+1}/G_i abelian. Galois: an equation is solvable by radicals iff its Galois group is solvable.