@article{EJDE_1998__1998__a4,
     author = {Joyal,  Pierre},
     title = {Invariance of {Poincar\'e-Lyapunov} polynomials under the group of rotations},
     journal = {Electronic journal of differential equations},
     year = {1998},
     volume = {1998},
     zbl = {0924.58089},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EJDE_1998__1998__a4/}
}