  A subalgebra B of a Lie algebra L is called a c-ideal of L if there is an
ideal C of L such that L = B + C and B \cap C \leq B_L, where B_L is the
largest ideal of $L$ contained in B. This is analogous to the concept of
c-normal subgroup, which has been studied by a number of authors. We obtain
some properties of c-ideals and use them to give some characterisations of
solvable and supersolvable Lie algebras. We also classify those Lie algebras in
which every one-dimensional subalgebra is a c-ideal.
