Symbols and orbits for 3-step nilpotent Lie groups

Sufficient conditions are derived for L²-boundedness of a convolution operator on a 3-step nilpotent Lie group. This is achieved by producing estimates on the Kirillov symbol of the operator, and is closely linked to the co-adjoint orbit structure of the group. A structure theorem for 3-step nilpotent Lie groups with 1-dimensional center is proved.