When is sum of first $n$ natural numbers a square?
44790
Find $n$ for which $\frac{n(n+1)}{2}$ is perfect square
143113
Non-inductive, not combinatorial proof of $\sum_{i \mathop = 0}^n \binom n i^2 = \binom {2 n} n$
52861
Natural Numbers and Well ordering
5692
Proving that there is a perfect square between $n$ and $2n$
80671
Product of sums of square is a sum of squares.
45392
Every natural number can be written as the sum of distinct Fibonacci numbers?
125702
There is a square $Q$ consisting of $(0,0), (2,0), (0,2), (2,2)$
22800
Can $\sqrt{n} + \sqrt{m}$ be rational if neither $n,m$ are perfect squares?
153594
When is $1^5 + 2^5 + \ldots + n^5$ a square?
24601
Proving that a natural number made entirely of 6's and 0's is not a square.
47471
The square of n+1-th prime is less than the product of the first n primes.
101832
Proof that sum of first $n$ cubes is always a perfect square
132541
Showing ${1\over n}\sum|S_i|=O(\sqrt n)$ for $S_i\subset [n]$, $|S_i\cap S_i|\le 1$ for $i\ne j$
104753
$\sum_{k=1}^nH_k = (n+1)H_n-n$. Why?
16472
Sum of First $n$ Squares Equals $\frac{n(n+1)(2n+1)}{6}$
10920
$\delta_2(n)$ the sum of the squares of the positive divisors of $n$.
46606
Can an integer of the form $4n+3$ written as a sum of two squares?