Question on checking the linearity of a differential operator

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Seydlitz
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Suppose I have this operator:

##D^2+2D+1##.

Is the ##1## there, when applied to a function, considered as identity operator?

Say:

##f(x)=x##.

Applying the operator results in:

##D^2(x)+2D(x)+(x)## or ##D^2(x)+2D(x)+1##?

If ##1## here is considered as an identity operator then the answer will be the former, and the whole operator is linear. But if it's the former, I don't see why the operator will be linear because of the extra ##1## term.

##D## refers to ##\frac{d}{dx}##, and the power of it refers to the order of the derivative.
 
on Phys.org
Ok thanks tiny-tim!