constructor() {

    		this.coefficients = [];

    		for ( let i = 0; i < 9; i ++ ) {

    			this.coefficients.push( new Vector3() );

    		}

    	}
Primary reference:
  https://graphics.stanford.edu/papers/envmap/envmap.pdf

Secondary reference:
  https://www.ppsloan.org/publications/StupidSH36.pdf