Find number of d's that satisfies $d, d+1, d+2... = N$ for an $N$
2137
Find the number of ways to express 1050 as sum of consecutive integers
98514
Prove by induction that the number of derangements of length $n$ is $D_n = (n-1)(D_{n-1}+D_{n-2}), n>2$
155372
Find the number of permutations in $S_n$ containing fixed elements in one cycle
19599
$ d(x_n,y_n) \to d(x,y)$ if $x_n\to x$ and $y_n \to y$
74924
identity element of $(D_n ,\gcd)$
24370
Dominos ($2 \times 1$ on $2 \times n$ and on $3 \times 2n$)
46814
$ a+b+c+d=6 , a^2+b^2+c^2+d^2=12$ $\implies$ $ 36 \leq 4(a^3+b^3+c^3+d^3)-(a^4+b^4+c^4+d^4) \leq48 $
106896
Number of solutions of $x_1+2x_2+\cdots+kx_k=n$?
107964
Simple combinatorics problem.
39621
Let a, b, c, d be rational numbers...
79568
Cardinality of the set $D$
87320
Proving identities like $\sum_{k=1}^nk{n\choose k}^2=n{2n-1\choose n}$ combinatorially
106416
If $n,m \in \mathbb{N}$ then there are $c,d$ such that $cd = (m,n)$, $(c,d) = 1$ and $(m/c,n/d) = 1$.
146491
Number of solutions of $x_1+x_2+\dots+x_k=n$ with $x_i\le r$
8446
Number of solutions over positive integers of $x_1 + x_2 + ... + x_n = S$ for $x_i \in [0, 25]$
39724
Combinatorial Identities
26087
Find all natural numbers $n > 1$ and $m > 1$ such that $1!3!5!\cdots(2n - 1)! = m!$