# Questions tagged [20-questions]

In the earliest days of MathOverflow, there was a '20 questions' seminar (see <http://sbseminar.wordpress.com/category/20-questions/>) run by graduate students at Berkeley. Many questions from the seminar were cross-posted to MathOverflow. This tag now exists solely for the historical record.

**11**

**3**answers

### How much “Morse theory” can be accomplished given only a continuous transformation of a space?

**21**

**4**answers

### Which sequences can be extended to analytic functions? (e. g., Ackermann's function)

**9**

**5**answers

### Analogues of the Weierstrass p function for higher genus compact Riemann surfaces

**15**

**3**answers

### Cohomology and Eilenberg-MacLane spaces

**11**

**4**answers

### When does the sheaf direct image functor f_* have a right adjoint?

**13**

**3**answers

### How should I approximate real numbers by algebraic ones?

**6**

**2**answers

### Can I finitely color Z^2 such that (x,a) and (a,y) are different for every x,y,a?

**8**

**6**answers

### What is an example of a topological space that is not homotopy equivalent to a CW-complex?

**70**

**9**answers

### understanding Steenrod squares

**7**

**2**answers

### Characterizing the Radon transforms of log-concave functions

**19**

**4**answers

### Can you explicitly write $\mathbb{R}^2$ as a disjoint union of two totally path disconnected sets?

**9**

**6**answers

### When does the sequence of iterates of a rational function converge?

**11**

**4**answers

### Can you describe the image of the exponential map B(H)->B(H).

**7**

**3**answers

### Is there a category in which finite limits and directed colimits *don't* commute

**26**

**7**answers

### How do you see the genus of a curve, just looking at its function field?

**7**

**2**answers

### Critical points on a fiber bundle

**7**

**2**answers

### How can you find small denominators inside triangles?

**45**

**5**answers