[1] Boyer C. P., Kalnins E. G., Miller W., Jr.: 
Symmetry and separation of variables for the Helmholtz and Laplace equations. Nagoya Math. J. 60 (1976), 35–80.  
MR 0393791 | 
Zbl 0314.33011[2] Cox D., Little J., O’Shea D.: 
Ideals, varieties, and algorithms. An introduction to computational algebraic geometry and commutative algebra. Second edition. Undergraduate Texts in Mathematics. Springer-Verlag, New York, 1997, xiv+536 pp.  
MR 1417938[4] Eastwood M., Leistner T.: 
Higher Symmetries of the Square of the Laplacian. preprint math.DG/0610610.  
MR 2384717 | 
Zbl 1137.58014[5] Fefferman C., Graham C. R.: The ambient metric. arXiv:0710.0919. 
[6] Gover A. R.: 
Laplacian operators and Q-curvature on conformally Einstein manifolds. Mathematische Annalen, 336 (2006), 311–334.  
MR 2244375 | 
Zbl 1125.53032[7] Gover A. R., Šilhan J.: 
Commuting linear operators and decompositions; applications to Einstein manifolds. Preprint math/0701377 , www.arxiv.org.  
MR 2585804 | 
Zbl 1195.47038[8] Graham C. R., Jenne R., Mason J. V., Sparling G. A.: 
Conformally invariant powers of the Laplacian, I: Existence. J. London Math. Soc. 46, (1992), 557–565.  
MR 1190438 | 
Zbl 0726.53010[9] Miller W., Jr.: 
Symmetry and separation of variables. Encyclopedia of Mathematics and its Applications, Vol. 4. Addison-Wesley Publishing Co., Reading, Mass.-London-Amsterdam, 1977, xxx+285 pp.   
MR 0460751 | 
Zbl 0368.35002