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1233dfv
  • Member for 10 years, 7 months
  • Last seen more than 2 years ago
72 votes

How can I evaluate $\sum_{n=0}^\infty(n+1)x^n$?

27 votes

What is the proof that the total number of subsets of a set is $2^n$?

19 votes
Accepted

Prove $\sqrt{2} + \sqrt{5}$ is irrational

16 votes

Why don't I get $e$ when I solve $\lim_{n\to \infty}(1 + \frac{1}{n})^n$?

12 votes

Pigeonhole Principle Question - Group of 6 people, do 3 either know each other or not?

11 votes

Subset of a countable set is itself countable

9 votes

Help finding a combinatorial proof of $k {n \choose k } = n {n - 1 \choose k -1}$

9 votes
Accepted

Why is $2^n$ the maximum number of subsets of a set of size $n$?

8 votes

Every planar graph has a vertex of degree at most 5.

8 votes

What five odd integers have a sum of $30$?

7 votes

the Nordhaus-Gaddum problems for chromatic number of graph and its complement

7 votes
Accepted

Combinatorics Pigeonhole problem

6 votes
Accepted

Chromatic Number

6 votes
Accepted

Is the differential equation $y'=x+y$ separable?

6 votes
Accepted

How to prove that $\sum_{k=0}^n \binom nk k^2=2^{n-2}(n^2+n)$

5 votes
Accepted

Proof by induction that alternating sum of binomial coefficients is $0$

5 votes

Find all complex solutions of $\sin(z)=1$

5 votes
Accepted

Proving an Combination formula $ \binom{n}{k} = \binom{n-1}{k}+\binom{n-1}{k-1}$

5 votes
Accepted

Minimum number of coins to ensure 10 coins of one type are selected

5 votes

Prove that if a $k$-regular bipartite graph has a bipartition $(x,y)$ then $\vert x\vert=\vert y \vert$

5 votes

Let $G$ be a graph of girth $5$ for which all vertices have degree $\geq d$. Show that $G$ has at least $d^2+1$ vertices.

5 votes
Accepted

Permutations and Derangements

4 votes
Accepted

Proof that the gamma function is an extension of the factorial function

4 votes

Inclusion-Exclusion Principle.

4 votes

Recurrence equations/General solutions

4 votes

Number of divisors of a number

4 votes

Prove that $\binom{n}{r} + \binom{n}{r+1} = \binom{n+1}{r+1} $

4 votes

Leisure reading for an undergraduate student

3 votes

Limit of $\left(1+\frac{a}{x}\right)^x$ with and without L'Hôpital's rule

3 votes

Derivatives question help

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