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In mathematics, a delta operator is a shift-equivariant linear operator Q : K ⟶ K {\displaystyle Q\colon \mathbb {K} \longrightarrow \mathbb {K} } on the vector space of polynomials in a variable x {\displaystyle x} over a field K {\displaystyle \mathbb {K} } that reduces degrees by one.
To say that Q {\displaystyle Q} is shift-equivariant means that if g = f {\displaystyle g=f} , then
In other words, if f {\displaystyle f} is a "shift" of g {\displaystyle g} , then Q f {\displaystyle Qf} is also a shift of Q g {\displaystyle Qg} , and has the same "shifting vector" a {\displaystyle a}.
To say that an operator reduces degree by one means that if f {\displaystyle f} is a polynomial of degree n {\displaystyle n} , then Q f {\displaystyle Qf} is either a polynomial of degree n − 1 {\displaystyle n-1} , or, in case n = 0 {\displaystyle n=0} , Q f {\displaystyle Qf} is 0.