When k[x,y] is flat over a certain subring?
		
		    Let k be a field of characteristic zero and let a,b,c,d belong to k[x,y]. Assume that k[x,y] is flat over R_1=k[a+c,b+d] and over R_2=k[a-c,b-d]. Is it true that k[x,y] is flat over R=k[a,b,d,c]?...		
		
		    
			20 December 2020
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