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 11h asked Characteristic of a pseudo-upper-triangular matrix. 1d comment Does $\sum_{n=2}^\infty \frac{n^2-4}{(n-1)^2(n+3)^2}$ converge? @Did: Wait, nvm. I see what you're saying.... I can't edit it I will just delete it. 1d accepted What can the operator $T$ be? 1d comment What can the operator $T$ be? This would imply that $T = P^{-1}DP$ for some diagonal matrix $D$. But the eigenvalues as I have shown must be $0$ so $D = 0$ hence $T = P^{-1}0P = 0$ so $T = 0$. Thanks. 1d asked What can the operator $T$ be? 2d comment Is there any relatively quick way to diagonalize this matrix with an orthogonal matrix? Yeah, sorry. I calculated the eigenvalues twice in my notebook and copied the wrong ones down. 2d comment Investigation Problem to challenge mathematical reasoning I don't know how "original" you need to be but this MO post mathoverflow.net/questions/48771/… seems to be related. Some of the proofs are a little above Calc 2, but you may be able to understand the first example: Euler's (not Euclid's) proof that their are infinitely primes. 2d accepted Is there any relatively quick way to diagonalize this matrix with an orthogonal matrix? 2d comment Is there any relatively quick way to diagonalize this matrix with an orthogonal matrix? I learned orthogonal as $A\cdot A^{T} = I_n$ and I wasn't really aware of the orthonormality that needed preservation. Normalizing each vector gives an identity matrix. Thanks! 2d asked Is there any relatively quick way to diagonalize this matrix with an orthogonal matrix? May 2 suggested rejected edit on Determine the derivative $\frac{dy}{dx}$ of the integral May 1 answered An induction problem that I can't think of an approach. May 1 comment Showing $A+B$ is invertible? Thanks. I guess I should have kept trying... At least I was on the right track after all. :) May 1 accepted Showing $A+B$ is invertible? May 1 asked Showing $A+B$ is invertible? Apr 29 suggested rejected edit on Prime number theory. Apr 28 revised Prove that the product of two invertible matrices also invertible added latex Apr 28 awarded Organizer Apr 28 suggested approved edit on Prove that the product of two invertible matrices also invertible Apr 28 revised The area of trapezium is given by $A=(a^2-x^2)(x+a)$. Find x for the area to be a maximum and find A max. fixed latex, also this is not abstract algebra.