For my homework, the official question reads: Find bases for the null space and range of $T:P_2({\Bbb R}) \rightarrow P_3({\Bbb R})$ given by $$(Tf)(x) = xf(x) - \int_0^x f(t) dt [sic]$$

Firstly, I suspect the $f(t)dt$ is a typo and am going to assume that it's $f(x) dx$. If this is incorrect and there's something I'm clueless about, this whole question is now pointless...

So assuming I'm working with $$(Tf)(x) = xf(x) - \int_0^x f(x) dx.$$

I figure $f(x) = a_1x^2 + a_2x+a_3$, so then $T(f(x))=(2/3)a_1x^3 + (1/2)a_2x^2$. Arbitrarily setting the $P_2$ vector to $\{1, x, x^2\}$, I calculate $$T(1) = (2/3)a_1+(1/2)a_2$$ $$T(x)=(2/3)a_1x^3+(1/2)a_2x$$ $$T(x^2)=(2/3)a_1x^6+(1/2)a_2x^2$$

So I should have this: $$\begin{bmatrix} (2/3)a_1+(1/2)a_2&0\\ 0&(1/2)a_2&0\\ 0&0&(1/2)a_2\\ 0&(2/3)a_1&0\\ 0&0&0\\ 0&0&(2/3)a_1\\ \end{bmatrix}$$

Putting that into a rref, I get all 3 columns (of $M_{5x3})$) as pivotal, and since $dim(P_3) = 4$ and $dim(R(T)) = 3$:$$\begin{bmatrix} 1&0&0\\ 0&1&0\\ 0&0&1\\ 0&0&0\\ \end{bmatrix}$$ So $x_1\begin{bmatrix}1\\0\\0\\0\end{bmatrix}+x_2\begin{bmatrix}0\\1\\0\\0\end{bmatrix}+x_3\begin{bmatrix}0\\0\\1\\0\end{bmatrix}$ right?

Now for my question: Did I do this all correctly? This should technically mean that my $dim(R(T))=3$, and since I am transposing into $P_3$, I should have $dim(N(T)) =1$... which would mean it'd just be $x_4\begin{bmatrix}\\0\\0\\0\\1\end{bmatrix}$?

  • $\begingroup$ It must be $\;f(t)\,dt\;$ : that second summand in that definition is a function of the upper limit of the integral .\ $\endgroup$
    – DonAntonio
    Sep 24, 2016 at 9:59
  • $\begingroup$ What is for you $\;P_n(\Bbb R)\;$ ? All the real polynomials of degree less than equal two or less than equal one ($1$) ? Both definitions exist pretty widely. $\endgroup$
    – DonAntonio
    Sep 24, 2016 at 10:00
  • $\begingroup$ I'm not quite sure what you mean by the P_n question? The professor has stated it's simply $\{x^0+x^1+x^2+...x^n\}$; is this what you mean? $\endgroup$
    – Asinine
    Sep 26, 2016 at 1:10

1 Answer 1


What you did seems correct (except the integral part):

$$f(x)=a_0+a_1x+a_2x^2\implies Tf=a_0x+a_1x^2+a_2x^3-\int_0^x(a_0+a_1t+a_2t^2)dt=$$



$$Tf=0\iff a_1=a_2=0\,,\,\,\text{so for example}\ker T=\text{Span}\,\{1\}$$

I'll leave it to you to find out the image of $\;T\;$ , but pay attention: you don't need at all to calculate the matrix of $\;T\;$ wrt some basis in the domain and codomain: that'd only be too much timeand work wasted and it is irrelevant for the question itself.

  • $\begingroup$ Ahhh, okay so I totally mucked up the integral. Thank you for that explanation as well. $\endgroup$
    – Asinine
    Sep 26, 2016 at 1:08

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