JohnD
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 Apr10 comment Understanding the definition of a set with $C^k$ boundary and of the outward pointing normal vector field How would you define the normal vector at the corners of the square? Think about what it means to be the normal vector there... And why a lack of $C^1$ is a problem... Apr10 comment Understanding the definition of a set with $C^k$ boundary and of the outward pointing normal vector field How about $U$ the interior of a square? Then the boundary of $U$ is continuous, but not smooth ($C^1$) at the corners. Apr8 awarded Enlightened Apr8 awarded Nice Answer Apr3 awarded Nice Answer Apr3 comment Find Transformation Matrix $T$ relative to new bases such that $T$ is in diagonal form This is the only approach I know of off the top of my head. Apr1 revised Find Transformation Matrix $T$ relative to new bases such that $T$ is in diagonal form added 8 characters in body Apr1 answered Find Transformation Matrix $T$ relative to new bases such that $T$ is in diagonal form Feb3 awarded Fanatic Jan21 reviewed Approve Inequality $(n!)^2\le \left[\frac{(n+1)(n+2)}{6}\right]^n$ Jan21 reviewed Approve Point of intersection closest to the origin Jan21 reviewed Approve How to solve the improper integral $\int_{-\infty}^{\infty} \frac{x^2}{x^6+9}dx$ (possible trig substitution) Jan21 reviewed Approve Differential Equation $y'+y\cdot\frac{1}{1+t^2}=1-y\cdot\frac{t^3}{1+t^4}$ Jan21 reviewed Reject How does constant affect the limit Jan21 reviewed Approve Finding the volume of revolution using the method of shells Jan19 comment Why is it that while taking the inverse matrix a Wronskian pops up in this solution? Glad to help. The more you learn/farther you go in mathematics you will see more and more connections between all the things you've already done. Jan19 answered Why is it that while taking the inverse matrix a Wronskian pops up in this solution? Jan8 reviewed No Action Needed Circle tangent to two other circles Jan8 reviewed No Action Needed Calculating area of complex ploygons (odd shapes) Jan8 reviewed No Action Needed Deduction Theorem and Closed Formulas