A purely algebraic construction of a gauge and renormalization group invariant scalar glueball operatorD. Dudal, S. P. Sorella, N. Vandersickel and H. VerscheldeEur. Phys. J. C, 64 1 (2009) 147-159DOI: https://doi.org/10.1140/epjc/s10052-009-1139-3