Prove that the (recursive) sum of the digits of any number multiplied by 9 is 9
3128
Why $9$ & $11$ are special in divisibility tests using decimal digit sums? (casting out nines & elevens)
114277
Proving in Discrete Structures Problem that $2^N = \binom{N}{0} + \binom{N}{1} + \binom{N}{2} + \dots + \binom{N}{N}$
146200
proof by induction: sum of binomial coefficients $\sum_{k=0}^n (^n_k) = 2^n$
136459
Prove ${n - 1 \choose k - 1} + {n - 2 \choose k - 1} + {n - 3 \choose k - 1} + \dots + {k - 1 \choose k - 1} = {n \choose k}$
58247
Prove $\sum_{k=0}^n \binom{n+k}{k}=\binom{2n+1}{n}$
45460
Proof verification of a number theory problem involving sequences.
93022
Prove that a given recursion sequence converges
70128
Prove that $(fg)^{(n)} = \sum_{k=0}^n \binom{n}{k}f^{(k)}(x)g^{(n-k)}(x)$
10875
proof of binomial identity involving double sum
105985
Prove that all even integers $n \neq 2^k$ are expressible as a sum of consecutive positive integers
126124
Why is the sum of the digits in a multiple of 9 also a multiple of 9?
87843
Combinatorial proof that $\frac{({10!})!}{{10!}^{9!}}$ is an integer
49687
Show that the number 9 divides the number $ m$ if and only if the sum of the digits of the number $ m $ is divisible by 9.
133905
Intuitive proof of $\sum_{k=1}^{n} \binom{n}{k} k^{k-1} (n-k)^{n-k} = n^n$
143501
Find all $n\in\mathbb N^+$ such that the sum of the digits of $5^n$ is equal to $2^n$.
72026
Induction proof of $\sum_{k=1}^{n} \binom n k = 2^n -1 $
36694
Sum of first n natural numbers proof