Is there any definition of the sum of $n$ summands, when $n$ is not a natural number?
118387
How to extend summations to fractional orders?
2402
Does $\sin n$ have a maximum value for natural number $n$?
65672
Proof of the summation $n!=\sum_{k=0}^n \binom{n}{k}(n-k+1)^n(-1)^k$?
17375
When is the limit of a sum equal to the sum of limits?
7785
Cardinality of sum-set
143069
What are the direct summands in $\mathbb{Z}^n$?
54442
Sum of $k {n \choose k}$ is $n2^{n-1}$
132264
Is there a (basic) way to understand why a limit cannot be brought inside of a sum, in certain situations?
103347
combinatorial proof of summation
53005
Properties of $\liminf$ and $\limsup$ of sum of sequences: $\limsup s_n + \liminf t_n \leq \limsup (s_n + t_n) \leq \limsup s_n + \limsup t_n$
131708
Sum of powers of natural numbers
89279
If $\sum a_n/k^n = 0$ for all $k$, then $a_n = 0$ for all $n$
123955
How to compute $\sum\limits_{k=0}^n (-1)^k{2n-k\choose k}$?
121685
Is there a bijection of the natural numbers which swaps $\frac{1}{n}$-summable subsets with $\frac{1}{\sqrt{n}}$-summable subsets?
148901
Can a composite number $n$ be the arithmetic mean of $\sigma(n)$ and $\varphi(n)$?
18064
Sum of reciprocals of numbers with certain terms omitted
77957
How is $\mathbb N$ actually defined?