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.