Tagged Questions

4
votes
3answers
102 views

Proving or Disproving the Sum in a Set

I am doing review questions for an exam and I am completely stumped on this particular question: Let A = {2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32}. Prove or disprove that if I select 10 distinct ...
2
votes
5answers
115 views

$x^2\mid27 \implies x\mid9$ : Prove

$x$ is given as a natural number. I was trying this by direct proof: assume $27\mid x^2$, then $x^2 = 27m \Longrightarrow x=3\cdot \sqrt{3m} \Longrightarrow \sqrt{3m}$ must be integer ...
4
votes
1answer
47 views

Verification of a proof involving Hausdorff max. principle and collections

I am given this proposition to prove which is a corollary to HMP(Hausdorff maximality principle). My two concerns are: 1. is my attempted proof correct? and 2. Is HMP applicable even when we're ...
4
votes
1answer
70 views

Proofs whose length depends on the input

This may be a question from proof theory, but I'm not sure, since I don't know any proof theory. What I will be asking about is what happens, if the length of a proof isn't fixed: I'm going to present ...
3
votes
3answers
135 views

If $n\equiv 2\pmod 3$, then $7\mid 2^n+3$.

In this (btw, nice) answer to Twin primes of form $2^n+3$ and $2^n+5$, it was said that: If $n\equiv 2\pmod 3$, then $7\mid 2^n+3$? I'm not familiar with these kind of calculations, so I'd like ...
3
votes
1answer
472 views

Justifying exchange of limits in a double sum - a dubious proof

It is a standard theorem (given in Rudin's Principles of Mathematical Analysis, page 175, and many other places), that if $\{a_{ij}\}$ is a doubly indexed sequence, and $$\sum_{j=1}^\infty |a_{ij}| = ...
2
votes
1answer
156 views

Found a simpler proof, now how do I know if it's original?

I've found a simpler proof for some identity/theorem, hypothetically speaking, of course ;) How do I know if it hasn't been done before? For important results it's fairly easy to find. By the way, I ...
3
votes
1answer
201 views

Is this a good proof of Wilson's theorem? — ($(n-1)!+1 \equiv_n 0$ iff n is prime)

Theorem: $(n - 1)! + 1 \equiv_n 0$ if and only if $n$ is prime. To prove that if $n$ is not prime this is not true is trivial, so I'm just interested in proving that this is true for all p: ...