Hans Lundmark

 325 Different methods to compute $\sum\limits_{k=1}^\infty \frac{1}{k^2}$ (Basel problem) 142 How do I get the square root of a complex number? 113 Derivative of sigmoid function $\sigma (x) = \frac{1}{1+e^{-x}}$ 95 An Introduction to Tensors 75 How can we sum up $\sin$ and $\cos$ series when the angles are in arithmetic progression?

### Reputation (40,588)

 +10 How can one intuitively think about quaternions? +10 What does it means by “integrable on the circle” +10 How do I evaluate the Clifford product in dimensions greater than 3? +10 An Introduction to Tensors

### Questions (2)

 8 Example of non-Krull integrally closed BFD? 5 Example of non-Noetherian non-UFD Krull domain?

### Tags (340)

 572 calculus × 135 222 integration × 74 298 linear-algebra × 86 214 notation × 23 278 ordinary-differential-equations × 140 213 trigonometry × 35 268 real-analysis × 66 195 sequences-and-series × 33 223 derivatives × 51 189 multivariable-calculus × 88

### Bookmarks (413)

 1294 Visually stunning math concepts which are easy to explain 653 Splitting a sandwich and not feeling deceived 523 Examples of patterns that eventually fail 458 “The Egg:” Bizarre behavior of the roots of a family of polynomials. 438 Integral $\int_{-1}^1\frac1x\sqrt{\frac{1+x}{1-x}}\ln\left(\frac{2\,x^2+2\,x+1}{2\,x^2-2\,x+1}\right) \mathrm dx$