Some answers I particularly enjoyed writing:
How to prove that a $3 \times 3$ Magic Square must have $5$ in its middle cell?, Mathematics, May 18, 2016.
How to reduce the low-rank matrix completion problem to integer programming?, Computer Science, April 18, 2017.
Finding the gradient of the restricted function in terms of the gradient of the original function, September 23, 2021.
Matrix exponential via Euler's formula, Mathematics, November 23, 2020.
Second derivative of $\det ( \mathrm I_n + t \mathrm A )$, Mathematics, June 28, 2018.
Toeplitz matrix nearest to a given (symmetric) matrix, MathOverflow, March 15, 2018.
Writing $(x^2 + y^2 + z^2)^2 - 3 ( x^3 y + y^3 z + z^3 x)$ as a sum of two squares of quadratic forms, Mathematics, September 3-8, 2017.
Profile picture courtesy of Erik Rauch. For rank-1-matrices, go here.