6 What's the proof that the Euler totient function is multiplicative? 4 When has one sufficiently mastered an area of mathematics? 4 help with logarithmic integration. 3 How to do integration of $∫\tan (x) dx$ 3 How to calculate derivative of $f(x) = \frac{1}{1-2\cos^2x}$?

### Reputation (3,225)

 +10 Example to show non-uniquness of wave equation +10 What's the proof that the Euler totient function is multiplicative? +10 What exacty is the role played by Jacobian or Wronskian? +10 When has one sufficiently mastered an area of mathematics?

### Questions (37)

 12 How to construct examples of functions in the Spaces of type $\mathcal{S}$ 6 What exacty is the role played by Jacobian or Wronskian? 6 Does the integral $\int_{a}^{b}\frac{dx}{\sqrt{(x-a)(x-b)}}$ exist? 6 What is $\inf\emptyset$ and $\sup\emptyset$? [duplicate] 5 Is there a difference between transform and transformation?

### Tags (123)

 30 calculus × 29 9 soft-question × 7 19 ordinary-differential-equations × 27 8 linear-algebra × 17 16 real-analysis × 18 7 partial-differential-equations × 15 13 integration × 14 7 elementary-number-theory × 4 9 trigonometry × 9 7 derivatives × 4

### Bookmarks (144)

 1363 Visually stunning math concepts which are easy to explain 676 Does $\pi$ contain all possible number combinations? 422 Proofs that every mathematician should know. [closed] 395 Fourier transform for dummies 348 Find five positive integers whose reciprocals sum to $1$