User Casteels - Mathematics Stack Exchange

Casteels

Member for 10 years, 1 month
Last seen more than a week ago

math.ucsb.edu/~casteels
Santa Barbara, CA

289 Answers

Score Activity Newest

97 votes

Accepted

Express 99 2/3% as a fraction? No calculator

answered Jun 9, 2015 at 15:11

54 votes

Proof that $2^{222}-1$ is divisible by 3

answered Mar 12, 2015 at 13:46

27 votes

Rational + irrational = always irrational?

answered Oct 28, 2013 at 13:44

24 votes

when is $n!+10$ a perfect square?

answered Feb 24, 2015 at 13:06

23 votes

How can I prove that one of $n$, $n+2$, and $n+4$ must be divisible by three, for any $n\in\mathbb{N}$

answered Oct 11, 2013 at 14:09

20 votes

Why is the multiset of eigenvalues called spectrum?

answered Nov 13, 2013 at 9:36

18 votes

Infinite Series $\sum\limits_{k=1}^{\infty}\frac{k^n}{k!}$

answered Sep 24, 2013 at 10:57

17 votes

Accepted

Solve $\phi(n) = 123456789 $ or prove there is no such $n$

answered Apr 20, 2015 at 12:37

16 votes

How to prove that for any graph on $8$ nodes with exactly $17$ edges has at least $4$ triangles.

answered Mar 4, 2014 at 13:42

12 votes

What is a fundamental cycle of a graph?

answered Oct 8, 2014 at 11:30

9 votes

Product of inverse matrices $ (AB)^{-1}$

answered Feb 24, 2014 at 10:03

9 votes

Accepted

Proof of binomial coefficient formula.

answered Nov 13, 2013 at 11:07

9 votes

Does $A^2B^2=I$ imply $AB=I$?

answered Jun 16, 2015 at 14:17

8 votes

Delete a vertex,how to show that graph remains connected?

answered Dec 30, 2013 at 17:45

8 votes

Accepted

Determinant of the inverse matrix

answered Jan 27, 2015 at 11:50

7 votes

3 points lie in the same straight line. Then why is this formula valid?

answered Feb 12, 2015 at 14:11

7 votes

Trigonometry and Proportions

answered Jun 16, 2015 at 12:43

6 votes

Accepted

Directed Multigraph or Directed Simple Graph?

answered Dec 16, 2014 at 10:13

6 votes

Accepted

How to prove this matrix inequality $\det(A+B)\ge 2^n\sqrt{\det(A)\det(B)}$

answered Dec 5, 2013 at 10:59

6 votes

There's always a smaller rational $x$ with $x^2\gt2$

answered Sep 30, 2013 at 9:36

6 votes

Accepted

All $k$-regular subgraphs of $K_{n,n}$ have a perfect matching: a proof without Hall's Marriage Theorem?

answered Jul 9, 2014 at 9:46

6 votes

Determining graph biconnection from degree sequence

answered Jun 17, 2015 at 7:54

5 votes

Determine cycle from adjacency matrix

answered Aug 27, 2014 at 15:34

5 votes

Combinatorial problem: Directed Acyclic Graph

answered Feb 12, 2014 at 11:19

5 votes

Accepted

Counting triangles in triangular graphs

answered Apr 11, 2014 at 9:16

5 votes

Accepted

Minimum area of the parallelepiped surface

answered Nov 19, 2013 at 10:56

5 votes

Accepted

Can every simple graph be embedded on a circuit board?

answered Dec 25, 2013 at 18:53

5 votes

To take notes or not to

answered Feb 4, 2014 at 10:26

5 votes

Accepted

Example showing ,for a given graph G , spanning trees need not be unique

answered Dec 17, 2014 at 10:49

5 votes

Accepted

Prove that a graph with the same number of edges and vertices contains one cycle

answered Jan 19, 2015 at 11:39

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