Determine for the numerical semigroup ring K [ U 3 , U 4 , U 5 , … ] {\displaystyle {}K[U^{3},U^{4},U^{5},\ldots ]} unitary differential operators for the elements U 3 , U 4 , U 5 {\displaystyle {}U^{3},U^{4},U^{5}} .